diff --git a/src/lib.rs b/src/lib.rs
index c248f702..c95000e5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -186,6 +186,7 @@ pub fn optimize(filepath: &Path, opts: &Options) -> Result<(), String> {
 
     if let Some(interlacing) = opts.interlace {
         if png.change_interlacing(interlacing) {
+            png.ihdr_data.interlaced = interlacing;
             something_changed = true;
         }
     }
diff --git a/src/png.rs b/src/png.rs
index 37607615..33a4b58d 100644
--- a/src/png.rs
+++ b/src/png.rs
@@ -156,11 +156,7 @@ impl<'a> Iterator for ScanLines<'a> {
             let mut bits_per_line = self.png.ihdr_data.width as usize * bits_per_pixel;
             let y_steps;
             match self.pass {
-                Some((1, _)) => {
-                    bits_per_line = (bits_per_line as f32 / 8f32).ceil() as usize;
-                    y_steps = 8;
-                }
-                Some((2, _)) => {
+                Some((1, _)) | Some((2, _)) => {
                     bits_per_line = (bits_per_line as f32 / 8f32).ceil() as usize;
                     y_steps = 8;
                 }
@@ -634,17 +630,225 @@ impl PngData {
     }
     /// Convert the image to the specified interlacing type
     /// Returns true if the interlacing was changed, false otherwise
+    /// The `interlace` parameter specifies the *new* interlacing mode
+    /// Assumes that the data has already been de-filtered
     pub fn change_interlacing(&mut self, interlace: u8) -> bool {
         if interlace == self.ihdr_data.interlaced {
             return false;
         }
 
         if interlace == 1 {
-            // TODO: Interlace
+            // Convert progressive to interlaced data
+            interlace_image(self);
         } else {
-            // TODO: Deinterlace
+            // Convert interlaced to progressive data
+            deinterlace_image(self);
         }
-        false
+        true
+    }
+}
+
+fn interlace_image(png: &mut PngData) {
+    let mut passes: Vec<BitVec> = Vec::with_capacity(7);
+    for _ in 0..7 {
+        passes.push(BitVec::new());
+    }
+    let bits_per_pixel = png.ihdr_data.bit_depth.as_u8() * png.channels_per_pixel();
+    for (index, line) in png.scan_lines().enumerate() {
+        match index % 8 {
+            // Add filter bytes to appropriate lines
+            0 => {
+                passes[0].extend(BitVec::from_elem(8, false));
+                passes[3].extend(BitVec::from_elem(8, false));
+                passes[5].extend(BitVec::from_elem(8, false));
+                if png.ihdr_data.width > 4 {
+                    passes[1].extend(BitVec::from_elem(8, false));
+                }
+            }
+            4 => {
+                passes[3].extend(BitVec::from_elem(8, false));
+                passes[5].extend(BitVec::from_elem(8, false));
+                passes[2].extend(BitVec::from_elem(8, false));
+            }
+            2 | 6 => {
+                passes[4].extend(BitVec::from_elem(8, false));
+                passes[5].extend(BitVec::from_elem(8, false));
+            }
+            _ => {
+                passes[6].extend(BitVec::from_elem(8, false));
+            }
+        }
+        let bit_vec = BitVec::from_bytes(&line.data);
+        for (i, bit) in bit_vec.iter().enumerate() {
+            // Avoid moving padded 0's into new image
+            if i >= (png.ihdr_data.width * bits_per_pixel as u32) as usize {
+                break;
+            }
+            // Copy pixels into interlaced passes
+            let pix_modulo = (((i / bits_per_pixel as usize) as f32).floor() as usize) % 8;
+            match index % 8 {
+                0 => {
+                    match pix_modulo {
+                        0 => passes[0].push(bit),
+                        4 => passes[1].push(bit),
+                        2 | 6 => passes[3].push(bit),
+                        _ => passes[5].push(bit),
+                    }
+                }
+                4 => {
+                    match pix_modulo {
+                        0 | 4 => passes[2].push(bit),
+                        2 | 6 => passes[3].push(bit),
+                        _ => passes[5].push(bit),
+                    }
+                }
+                2 | 6 => {
+                    match pix_modulo % 2 {
+                        0 => passes[4].push(bit),
+                        _ => passes[5].push(bit),
+                    }
+                }
+                _ => {
+                    passes[6].push(bit);
+                }
+            }
+        }
+        // Pad end of line on each pass to get 8 bits per byte
+        for pass in &mut passes {
+            while pass.len() % 8 != 0 {
+                pass.push(false);
+            }
+        }
+    }
+    let mut output = Vec::new();
+    for pass in &passes {
+        output.extend(pass.to_bytes());
+    }
+    png.raw_data = output;
+}
+
+fn deinterlace_image(png: &mut PngData) {
+    let bits_per_pixel = png.ihdr_data.bit_depth.as_u8() * png.channels_per_pixel();
+    let mut lines: Vec<BitVec> = Vec::with_capacity(png.ihdr_data.height as usize);
+    for _ in 0..png.ihdr_data.height {
+        // Initialize each output line with a starting filter byte of 0
+        // as well as some blank data
+        lines.push(BitVec::from_elem(8 + bits_per_pixel as usize * png.ihdr_data.width as usize,
+                                     false));
+    }
+    let mut current_pass = 1;
+    let mut pass_constants = interlaced_constants(current_pass);
+    let mut current_y: usize = pass_constants.y_shift as usize;
+    for line in png.scan_lines() {
+        let bit_vec = BitVec::from_bytes(&line.data);
+        let bits_in_line = ((png.ihdr_data.width - pass_constants.x_shift as u32) as f32 /
+                            pass_constants.x_step as f32)
+                               .ceil() as usize *
+                           bits_per_pixel as usize;
+        for (i, bit) in bit_vec.iter().enumerate() {
+            // Avoid moving padded 0's into new image
+            if i >= bits_in_line {
+                break;
+            }
+            let current_x: usize = pass_constants.x_shift as usize +
+                                   (i / bits_per_pixel as usize) * pass_constants.x_step as usize;
+            // Copy this bit into the output line, offset by 8 because of filter byte
+            let index = 8 + (i % bits_per_pixel as usize) + current_x * bits_per_pixel as usize;
+            lines[current_y].set(index, bit);
+        }
+        // Calculate the next line and move to next pass if necessary
+        current_y += pass_constants.y_step as usize;
+        if current_y >= png.ihdr_data.height as usize {
+            if current_pass == 7 {
+                break;
+            }
+            current_pass += 1;
+            if current_pass == 2 && png.ihdr_data.width <= 4 {
+                current_pass += 1;
+            }
+            if current_pass == 3 && png.ihdr_data.height <= 4 {
+                current_pass += 1;
+            }
+            pass_constants = interlaced_constants(current_pass);
+            current_y = pass_constants.y_shift as usize;
+        }
+    }
+    let mut output = Vec::new();
+    for line in &mut lines {
+        while line.len() % 8 != 0 {
+            line.push(false);
+        }
+        output.extend(line.to_bytes());
+    }
+    png.raw_data = output;
+}
+
+struct InterlacedConstants {
+    x_shift: u8,
+    y_shift: u8,
+    x_step: u8,
+    y_step: u8,
+}
+
+fn interlaced_constants(pass: u8) -> InterlacedConstants {
+    match pass {
+        1 => {
+            InterlacedConstants {
+                x_shift: 0,
+                y_shift: 0,
+                x_step: 8,
+                y_step: 8,
+            }
+        }
+        2 => {
+            InterlacedConstants {
+                x_shift: 4,
+                y_shift: 0,
+                x_step: 8,
+                y_step: 8,
+            }
+        }
+        3 => {
+            InterlacedConstants {
+                x_shift: 0,
+                y_shift: 4,
+                x_step: 4,
+                y_step: 8,
+            }
+        }
+        4 => {
+            InterlacedConstants {
+                x_shift: 2,
+                y_shift: 0,
+                x_step: 4,
+                y_step: 4,
+            }
+        }
+        5 => {
+            InterlacedConstants {
+                x_shift: 0,
+                y_shift: 2,
+                x_step: 2,
+                y_step: 4,
+            }
+        }
+        6 => {
+            InterlacedConstants {
+                x_shift: 1,
+                y_shift: 0,
+                x_step: 2,
+                y_step: 2,
+            }
+        }
+        7 => {
+            InterlacedConstants {
+                x_shift: 0,
+                y_shift: 1,
+                x_step: 1,
+                y_step: 2,
+            }
+        }
+        _ => unreachable!(),
     }
 }
 
diff --git a/tests/files/interlacing_0_to_1_small_files.png b/tests/files/interlacing_0_to_1_small_files.png
new file mode 100644
index 00000000..576b88c9
Binary files /dev/null and b/tests/files/interlacing_0_to_1_small_files.png differ
diff --git a/tests/files/interlacing_1_to_0_small_files.png b/tests/files/interlacing_1_to_0_small_files.png
new file mode 100644
index 00000000..91af6176
Binary files /dev/null and b/tests/files/interlacing_1_to_0_small_files.png differ
diff --git a/tests/flags.rs b/tests/flags.rs
index 782e9d0d..3666809d 100644
--- a/tests/flags.rs
+++ b/tests/flags.rs
@@ -341,3 +341,85 @@ fn interlacing_1_to_0() {
 
     remove_file(output).ok();
 }
+
+#[test]
+fn interlacing_0_to_1_small_files() {
+    let input = PathBuf::from("tests/files/interlacing_0_to_1_small_files.png");
+    let mut opts = get_opts(&input);
+    opts.interlace = Some(1);
+    let output = opts.out_file.clone();
+
+    let png = png::PngData::new(&input).unwrap();
+
+    assert!(png.ihdr_data.interlaced == 0);
+    assert!(png.ihdr_data.color_type == png::ColorType::Indexed);
+    assert!(png.ihdr_data.bit_depth == png::BitDepth::Eight);
+
+    match oxipng::optimize(&input, &opts) {
+        Ok(_) => (),
+        Err(x) => panic!(x.to_owned()),
+    };
+    assert!(output.exists());
+
+    let png = match png::PngData::new(&output) {
+        Ok(x) => x,
+        Err(x) => {
+            remove_file(output).ok();
+            panic!(x.to_owned())
+        }
+    };
+
+    assert!(png.ihdr_data.interlaced == 1);
+    assert!(png.ihdr_data.color_type == png::ColorType::Indexed);
+    assert!(png.ihdr_data.bit_depth == png::BitDepth::One);
+
+    let old_png = image::open(&input).unwrap();
+    let new_png = image::open(&output).unwrap();
+
+    // Conversion should be lossless
+    assert!(old_png.pixels().map(|x| x.2.channels().to_owned()).collect::<Vec<Vec<u8>>>() ==
+            new_png.pixels().map(|x| x.2.channels().to_owned()).collect::<Vec<Vec<u8>>>());
+
+    remove_file(output).ok();
+}
+
+#[test]
+fn interlacing_1_to_0_small_files() {
+    let input = PathBuf::from("tests/files/interlacing_1_to_0_small_files.png");
+    let mut opts = get_opts(&input);
+    opts.interlace = Some(0);
+    let output = opts.out_file.clone();
+
+    let png = png::PngData::new(&input).unwrap();
+
+    assert!(png.ihdr_data.interlaced == 1);
+    assert!(png.ihdr_data.color_type == png::ColorType::Indexed);
+    assert!(png.ihdr_data.bit_depth == png::BitDepth::Eight);
+
+    match oxipng::optimize(&input, &opts) {
+        Ok(_) => (),
+        Err(x) => panic!(x.to_owned()),
+    };
+    assert!(output.exists());
+
+    let png = match png::PngData::new(&output) {
+        Ok(x) => x,
+        Err(x) => {
+            remove_file(output).ok();
+            panic!(x.to_owned())
+        }
+    };
+
+    assert!(png.ihdr_data.interlaced == 0);
+    assert!(png.ihdr_data.color_type == png::ColorType::Indexed);
+    assert!(png.ihdr_data.bit_depth == png::BitDepth::One);
+
+    let old_png = image::open(&input).unwrap();
+    let new_png = image::open(&output).unwrap();
+
+    // Conversion should be lossless
+    assert!(old_png.pixels().map(|x| x.2.channels().to_owned()).collect::<Vec<Vec<u8>>>() ==
+            new_png.pixels().map(|x| x.2.channels().to_owned()).collect::<Vec<Vec<u8>>>());
+
+    remove_file(output).ok();
+}