I have heard of people boiling them for a few minutes to sterilize them. I used to do it, but it was a pain to have to stand there and turn the cups so they got completely sterilized. Now I just put them in the microwave for a little over a minute. I sterilize (microwave) the cups, the measuring cup, and the netting (the shower spoof things) that I use in place of excelsior. I don't know if anyone else uses this method, but it seems to be working out fine for me.