Join Date: Sep 2004
Location: North Central Massachusetts
Bikes: Cannondale R600
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I agree with DMF - mostly likely a tension adjustment. If your chain skipped on the same gear, that's different. The only other thing that might be off is if the derailleur is not straight - if you have inadvertently pushed it out of alignment, this can wreak havoc as well.
Just because it's simple to turn the barrel adjuster doesn't mean that it's always easy to get right - you need some patience. Most rear derailleurs have a barrel adjuster that you can see from the rear. If you turn it counterclockwise, it tightens the tension (obviously the other direction loosens it). Usually, you start with it on the big chainring and the smallest cog. Shift once. If it doesn't make the shift at all, it's too loose, and you should really remove the slack from the cable (there's a pinch bolt that holds the cable). If it's too tight, then it will either shift two cogs, or more likely it will shift one cog and make noise because it's rubbing against the next cog. One technique is to tighten it until you can hear the noise of it being too tight, and then loosen it as you pedal until the noise just goes away.
Has it always slipped?