Shifters / Brake Levers

This is the first and most important consideration of a drivetrain package.

All new shifters are indexed systems meaning that one click on the shifter changes one gear. Each brand has a unique index that is only compatible to its own brand. In other words, your Shimano levers will only work with Shimano derailleurs. Same with Campy and same with SRAM. 

Additionally, 11 speed shifters will only work with 11 speed cassettes, 10 speed shifters with 10 speed cassettes, and so on.

However, typically brand models will be interchangeable. In other words, you can use Shimano Tiagra shifters with a Shimano Dura-Ace derailleur.