I started with newspaper. It's great, but I got tired of changing paper after they inevitably dumped fresh water. I switched then to shredded aspen. Had a male breeder get some of that stuck up in his vent and after a trip to the vet I got rid of that pointy splintery stuff. Then I tried indented kraft paper from Uline. It's nice, thick paper but again I eventually got tired of changing so much paper. It only soaks up so much before it has to be replaced. Then I tried cypress mulch. The only kind I could find around here was a blend, and I didn't like that it looked like there was potting soil mixed in. Water bowl + soil = mud. No thanks! I finally settled on sani chips from Harlan teklad, and am happy with those. For now. They seem to hold humidity pretty well, and poops/urates are easily scooped out. I change the whole tub once a month.