What do I need to use speed lights?

I currently use a Nikon D5100 but am soon upgrading to the D600. I want to be able to use several speed lights. I know I will need a transmitter, but not sure what else and how they go together.

You have 2 choices;
- you use Nikon speedlights - SB700 or SB910 will do Nikons Advanced Wireless, Creative Lighting system (CLS) - this allows not only TTL (auto flash metering) but you can also control the power out put of the flashes (in groups, A, B & C) from the in camera menu The pop-up flash acts as a trigger. This is a feature of the D600, but not your current D5100. Nikon CLS is an optical-based triggering system, so it can run into transmission difficulties - it's also expensive (a Nikon SB910 will set you back around £320).

- you use cheaper manual flash and radio triggers. This will give you control over flash output - BUT you'd need to manually dial in output on the back of each flash unit - but once done, it's more versatile than the CLS system. No TTL. Its much cheaper too - I use Yongnuo YN560's - about £50 each and RF602 radio triggers (more reliable than optical triggers) - £20 a set.

Having used both Nikon CLS with my Nikon speedlights (SB600 & SB800's) I now use almost exclusively manual flash and radio triggers.

This is a Strobist swim shoot using 2x YN560's & RF602's;

but my photostream also includes a lot of off camera flash work (details are given in descriptions);

I agree with DeepBlue.

I use a bunch of YN460's with some cheap radio triggers, You'll also want some lightstands and umbrella mounts.

Example shots:

Basically, you'll put a receiver unit under the flash and put a transmitter unit on your camera. The transmitter will tell the flashes to fire when you press the shutter release. Remember not to set your shutter speed higher than your flash synch speed (usually around 1/200th max) or you'll end up with a large black band.