A staple cartridge assembly, comprising: a cartridge body, comprising: a deck; and a plurality of staples movable between unfired positions and fired positions, said staples comprising staple legs and staple bases; and a layer configured to be captured by said staples, wherein said layer is a tissue thickness compensator, comprising: a tissue contacting surface; a plurality of first portions comprising a first thickness; and a plurality of second portions comprising a second thickness; wherein said first thickness is different from said second thickness, wherein said first portions are configured to be captured between said staple legs and said staple bases when said staples are in their fired positions, wherein said second portions are configured and arranged such that they are not captured between said staple legs and said staple bases when said staples are in their fired positions, characterised in that said tissue contacting surface comprises a plurality of cleats configured to reduce tissue slippage relative to said layer and in that said first portions comprise protrusions configured to be captured between said staple legs and said staple bases when said staples are in their fired positions. a cartridge body, comprising: a deck; and a plurality of staples movable between unfired positions and fired positions, said staples comprising staple legs and staple bases; and a deck; and a plurality of staples movable between unfired positions and fired positions, said staples comprising staple legs and staple bases; and a layer configured to be captured by said staples, wherein said layer is a tissue thickness compensator, comprising: a tissue contacting surface; a plurality of first portions comprising a first thickness; and a plurality of second portions comprising a second thickness; wherein said first thickness is different from said second thickness, wherein said first portions are configured to be captured between said staple legs and said staple bases when said staples are in their fired positions, wherein said second portions are configured and arranged such that they are not captured between said staple legs and said staple bases when said staples are in their fired positions, characterised in that said tissue contacting surface comprises a plurality of cleats configured to reduce tissue slippage relative to said layer and in that said first portions comprise protrusions configured to be captured between said staple legs and said staple bases when said staples are in their fired positions. a tissue contacting surface; a plurality of first portions comprising a first thickness; and a plurality of second portions comprising a second thickness; wherein said first thickness is different from said second thickness, wherein said first portions are configured to be captured between said staple legs and said staple bases when said staples are in their fired positions, wherein said second portions are configured and arranged such that they are not captured between said staple legs and said staple bases when said staples are in their fired positions, characterised in that said tissue contacting surface comprises a plurality of cleats configured to reduce tissue slippage relative to said layer and in that said first portions comprise protrusions configured to be captured between said staple legs and said staple bases when said staples are in their fired positions.