I keep three 500ml bottles of water and a 1liter larger bottle and replace them every so often. Like paul, I use one every so often if Im really thristy or I need to clean up a spill or something and I've never had any problem with them. As for food, its less of a concern in a emergancy (unless your up to some whacky stuff <img src="/images/graemlins/tongue.gif" alt="" />) so I just have three granola bars in my BOB there that I swap out every so often. I keep a few chemical light sticks in there and replace them in about 4 months cycles. Not so much that the chemical expires, just in case they were triggered by a sharp bump or what have you and have wasted away in their package. Other than that, I can't think of anything I replace or spoils. This is all in the trunk of a 94 shadow (don't laugh, we got a super deal on her) mind you not a exposed flat bed or anything.