Thick memory foam, very comfortable! Be careful where you let it “open up!” It comes all rolled up, and it was a little frozen (it was outside longer than I thought) so I left it in a room with the door shut to warm up and unroll (so my dogs didn’t get at it), and it unrolled right into the door and I almost couldn’t get into the room! I will also need to get some new deeper sheets as my regular ones are not deep enough to cover the topper and my mattress (this makes sense and I’m not mad at it).