diff --git a/src/lib.rs b/src/lib.rs
index 98384ab..135cfa2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -28,7 +28,7 @@
 
 pub use error::Error;
 pub use jpeg::get_exif_attr as get_exif_attr_from_jpeg;
-pub use tag::Tag;
+pub use tag::{Context, Tag};
 pub use tiff::Field;
 pub use tiff::parse_exif;
 pub use value::Value;
@@ -41,7 +41,7 @@ mod tmacro;
 mod endian;
 mod error;
 mod jpeg;
-mod tag;
+pub mod tag;
 mod tiff;
 mod util;
 mod value;
diff --git a/src/tag.rs b/src/tag.rs
index 432d5d2..26e59f2 100644
--- a/src/tag.rs
+++ b/src/tag.rs
@@ -24,6 +24,15 @@
 // SUCH DAMAGE.
 //
 
+//! Compatibility warning: Exif tag constants in this module will be
+//! converted to associated constants of Tag when the feature is
+//! stabilized.
+
+/// A tag of a TIFF field.
+///
+/// Use `exif::Tag` instead of `exif::tag::Tag`.  They are the same,
+/// but `exif::tag` will become private in the future versions.
+//
 // This is not an enum to keep safety and API stability, while
 // supporting unknown tag values.  This comment is based on the
 // behavior of Rust 1.12.
@@ -34,7 +43,82 @@
 // tends to break backward compatibility.  When Tag::VariantFoo is
 // defined in a new version of the library, the old codes using
 // Tag::Unknown(Foo's value) will break.
+//
+// Use of constants is restricted in patterns.  As of Rust 1.12,
+// PartialEq and Eq need to be _automatically derived_ for Tag to
+// emulate structural equivalency.
+// <https://github.com/rust-lang/rfcs/pull/1445>
+#[derive(Debug, PartialEq, Eq)]
+pub struct Tag(pub Context, pub u16);
 
-/// A tag of a TIFF field.
-#[derive(Debug)]
-pub struct Tag(pub u16);
+impl Tag {
+    /// Returns the context of the tag.
+    #[inline]
+    pub fn context(&self) -> Context {
+        self.0
+    }
+
+    /// Returns the value of the tag.
+    #[inline]
+    pub fn value(&self) -> u16 {
+        self.1
+    }
+}
+
+/// An enum that indicates how a tag value is interpreted.
+///
+/// Use `exif::Context` instead of `exif::tag::Context`.  They are the
+/// same, but `exif::tag` will become private in the future versions.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum Context {
+    /// TIFF attributes defined in the TIFF Rev. 6.0 specification.
+    Tiff,	// 0th IFD
+    /// Exif attributes.
+    Exif,	// 0th IFD -- Exif IFD
+    /// GPS attributes.
+    Gps,	// 0th IFD -- GPS IFD
+    /// Interoperability attributes.
+    Interop,	// 0th IFD -- Exif IFD -- Interoperability IFD
+    /// TIFF fields in the 1st IFD, which represents the thumbnail image.
+    Thumb,	// 1st IFD
+}
+
+macro_rules! generate_well_known_tag_constants {
+    (
+        $(
+            // Copy the doc attribute to the actual definition.
+            $( #[$attr:meta] )*
+            ($name:ident, $ctx:ident, $num:expr, $desc:expr)
+        ),+,
+    ) => (
+        $(
+            $( #[$attr] )*
+            #[allow(non_upper_case_globals)]
+            pub const $name: Tag = Tag(Context::$ctx, $num);
+        )+
+    )
+}
+
+// Tag constant names do not follow the Rust naming conventions but
+// the Exif field names: camel cases and all-capital acronyms.
+generate_well_known_tag_constants!(
+    // Exif-specific IFDs [EXIF23 4.6.3].
+
+    /// A pointer to the Exif IFD.  This is used for the internal structure
+    /// of Exif data and will not be returned to the user.
+    (ExifIFDPointer, Tiff, 0x8769, "Exif IFD pointer"),
+    /// A pointer to the GPS IFD.  This is used for the internal structure
+    /// of Exif data and will not be returned to the user.
+    (GPSInfoIFDPointer, Tiff, 0x8825, "GPS Info IFD pointer"),
+    /// A pointer to the interoperability IFD.  This is used for the internal
+    /// structure of Exif data and will not be returned to the user.
+    (InteropIFDPointer, Exif, 0xa005, "Interoperability IFD pointer"),
+
+    // TIFF attributes [EXIF23 4.6.4].
+
+    // Exif IFD attributes [EXIF23 4.6.5].
+
+    // GPS attributes [EXIF23 4.6.6].
+
+    // Interoperability attributes [EXIF23 4.6.7].
+);
diff --git a/src/tiff.rs b/src/tiff.rs
index 23f0b98..eb027f6 100644
--- a/src/tiff.rs
+++ b/src/tiff.rs
@@ -26,7 +26,8 @@
 
 use endian::{Endian, BigEndian, LittleEndian};
 use error::Error;
-use tag::Tag;
+use tag;
+use tag::{Context, Tag};
 use value::Value;
 use value::get_type_info;
 
@@ -62,11 +63,11 @@ fn parse_exif_sub<E>(data: &[u8])
         return Err(Error::InvalidFormat("Invalid forty two"));
     }
     let ifd_offset = E::loadu32(data, 4) as usize;
-    parse_ifd::<E>(data, ifd_offset)
+    parse_ifd::<E>(data, ifd_offset, Context::Tiff)
 }
 
 // Parse IFD [EXIF23 4.6.2].
-fn parse_ifd<E>(data: &[u8], offset: usize)
+fn parse_ifd<E>(data: &[u8], offset: usize, ctx: Context)
                 -> Result<Vec<Field>, Error> where E: Endian {
     // Count (the number of the entries).
     if data.len() < offset || data.len() - offset < 2 {
@@ -98,17 +99,57 @@ fn parse_ifd<E>(data: &[u8], offset: usize)
             }
             val = parser(data, ofs, cnt);
         }
-        fields.push(Field { tag: Tag(tag), value: val });
+
+        // No infinite recursion will occur because the context is not
+        // recursively defined.
+        // XXX Should we check the type and count of a pointer?
+        // A pointer field has type == LONG and count == 1, so the
+        // value (IFD offset) must be embedded in the "value offset"
+        // element of the field.
+        let tag = Tag(ctx, tag);
+        if tag == tag::ExifIFDPointer {
+            let mut v = try!(parse_ifd::<E>(data, ofs, Context::Exif));
+            fields.append(&mut v);
+        } else if tag == tag::GPSInfoIFDPointer {
+            let mut v = try!(parse_ifd::<E>(data, ofs, Context::Gps));
+            fields.append(&mut v);
+        } else if tag == tag::InteropIFDPointer {
+            let mut v = try!(parse_ifd::<E>(data, ofs, Context::Interop));
+            fields.append(&mut v);
+        } else {
+            fields.push(Field { tag: tag, value: val });
+        }
     }
 
     // Offset to the next IFD.
     if data.len() - offset - 2 - count * 12 < 4 {
         return Err(Error::InvalidFormat("Truncated IFD"));
     }
-    let next_ifd_offset = E::loadu32(data, offset + 2 + count * 12);
+    let next_ifd_offset = E::loadu32(data, offset + 2 + count * 12) as usize;
     if next_ifd_offset != 0 {
-        unimplemented!();
+        if ctx != Context::Tiff {
+            return Err(Error::InvalidFormat("Unexpected next IFD"));
+        }
+        let mut v = try!(
+            parse_ifd::<E>(data, next_ifd_offset, Context::Thumb));
+        fields.append(&mut v);
     }
 
     Ok(fields)
 }
+
+#[cfg(test)]
+mod tests {
+    use error::Error;
+    use super::*;
+
+    // Before the error is returned, the IFD is parsed twice as the
+    // 0th and 1st IFDs.
+    #[test]
+    fn inf_loop_by_next() {
+        let data = b"MM\0\x2a\0\0\0\x08\
+                     \0\x01\x01\0\0\x03\0\0\0\x01\0\x14\0\0\0\0\0\x08";
+        assert_err_pat!(parse_exif(data),
+                        Error::InvalidFormat("Unexpected next IFD"));
+    }
+}