Very little. You need to install the bar/stem assembly (all the cables are already hooked up), install the front wheel. Then it's just making any adjustments for fit (seatpost, stem) and brake or shifting adjustments needed (derailleurs, shifters, brake calipers), pump up the tires, and be on your way. I find that the shifting & braking is usually set up so that you don't need to adjust it much if at all, but it should be checked. It takes 20 minutes if you know what you're doing. Maybe an hour if you're not too familiar with it, but are somewhat mechanically inclined.