Canon's press material for the EOS 5D states that it 'defines (a) new D-SLR category', while we're not typically too concerned with marketing talk this particular statement is clearly pretty accurate. The EOS 5D is unlike any previous digital SLR in that it combines a full-frame (35 mm sized) high resolution sensor (12.8 megapixels) with a relatively compact body (slightly larger than the EOS 20D, although in your hand it feels noticeably 'chunkier'). The EOS 5D is aimed to slot in between the EOS 20D and the EOS-1D professional digital SLR's, an important difference when compared to the latter is that the EOS 5D doesn't have any environmental seals.