The drivetrain is comprised of the chain, crankset and derailleurs.

Problems shifting?  One major cause is a worn out chain.  Bicycling Magazine recommends a new chain every 1,500 miles in order to keep your bike running like new. Leaving a worn out chain on your bike will cause your cassette and chain rings to wear out faster as well. 

Here’s how to check to see if your chain needs replacement.

Although this chain is filthy, you can see that the pin lines up
exactly with the foot mark, which means that the chain is
hardly worn at all and well worth cleaning.

Check the chain for wear. If you’ve logged mega miles, the chain may be worn out. If so, rather than cleaning, you should replace it. To check, shift the chain onto the large chainring. Now, rest a ruler alongside the lower run of chain and see if you can measure exactly 12 inches between two pins (photo). If so, your drivetrain is okay. If the second pin exceeds the 12-inch mark by 1/8 inch or more, your chain is worn out and should be replaced. Keep in mind that if you use a new chain with a worn cassette, you may experience "skipping" while pedaling, which could cause a crash. This means you should replace the cassette, too.

Replace your chain as needed.  If your chain is worn out, bring your bicycle into Gus' for a professional replacement.