As the title says, I need to reinstall my tail-light lens and I am wondering what the best adhesive/sealant is to use. There was some fairly stiff white sealant on there, but it must have dried out over the years, 'cause the lens kept coming away from the housing. I cleaned it all off, and am ready to re-install. Any ideas are appreciated.

Silicone caulk (clear) works just fine.

Sorry gnosez but I have to disagree, clear silicone sealant is great for kitchen & bath but for automotive you need something that will take heat/cold, vibration, paint and petroleum products. Also, silicone isn't much of an adhesive. Permetex makes an extensive line of sealants for every application, a little more expensive you are only going to do it once.