What you need to know before buying any of these:- The flash control is manual only. Be ready to go around your flashes to adjust. Considering the price it is absolutely acceptable and expected. (Come on, Yongnuo, make me lie and release an ETTL version for the same price, I'll buy 6 immediately!)- The RF-603 does not feature a locking mechanism on the hot shoe attaching to the camera. I have nearly crashed a 580 Ex-II connected on top of one of these thanks to this omission. This is the only minus I am willing to retain considering the price, although admittedly in the majority of cases the cold-shoe you attach the RF-603 to will have such locking mechanism.- The RF-603 does not have master and slave units: they are all equal in functionality and can all be used as master or slave or to trigger the camera remotely (a very nice feature).- Up to 16 photographers can be controlling their flash with RF-603 at the same time without interfering with each other provided they set theirs to different channels. The channel is set on a DIP 4 switch in the battery compartment and all RF-603 that need to communicate need to be on the same channel. For now, just pick any combination and assign it to all your RF-603.- HOWEVER the RF-603 are not all the same. They behave the same, but there are flavors: the RF-603 N (for Nikon) and the RF-603 C (for Canon) have a different pin-out for the contacts in the hot-shoe, simply because the Canon and Nikon hot shoe contacts are different.You will need the right contacts for the camera you use - however normally it should not matter with what flash you use them as only the trigger is used and this one is the same regardless of the brand.- Then there is the matter of the remote control of the camera. Here the problem is that not all Canon cameras use the same plug for the shutter release (the one you use in pause B for Bulb). This determines the nature of the cable that comes with your pair of RF-603 C: RF-603 C1 has the cable for the rebel etc, and RF-603 C3 has the cable for the 7D, 5D, 1D etc.So, make sure you buy the set you need for your camera brand and model.How you use them:- Make sure all the RF-603 are on the same channel (set in a DIP-switch in the battery compartment).- Switch on all the RF-603 and THEN attach the flash you want on the top hot-shoe. Some people complain that the power switch is badly placed; it doesn't bother me.- If you have studio strobes, you can connect each to the PC sync connector at the back of a RF-603 with a cable that you will have to procure separately.- Connect a RF-603 on your camera hot shoe and if you want to control the camera remotely, plug the shutter release cable between the 603 and the camera.That's it.Pressing the shutter on any RF-603 within range and on the same channel will take a picture, firing all the flash attached to any RF-603 on that channel within range.That includes a RF-603 not connected to anything, so if you have a spare you can trigger remotely the camera and flash without a problem.If it does not work, you can check the following:- Do all 603 have fresh batteries?- Are they all switched on?- Are they all on the same channel?- Is the one connected to the camera plugged in on the hot-shoe?- Is it connected to the bulb port? (Only necessary to remote control the camera)- Ar they all in range? (Yongnuo claims 100m but I wonder under what circumstances I would need that much...)- Do you have more than one remote controlled camera?I have two sets: a RF-603 C1 and a RF-603 C3 to have a shutter release cable suitable for each of my cameras. All transceivers are identical and interchangeable and interact very nicely; no problem at all. I didn't even have to read the doc as I had picked up how to use these from the reviews here on Amazon!The one thing that does not work is remote controlling two or more cameras at the same time: the first one to shoot gets the flash; the other one is left in the dark. This is expected and normal. As long as you have no more than one camera on remote control you are fine.Someone was complaining that the master RF-603 (attached to the camera) still had to be on the hot shoe of the camera in addition to being connected to the bulb port.This is expected: not all cameras will trigger at the same moment when the shutter is pressed; for instance if you are in AF, your camera will want to focus before releasing the shutter and that may take a variable duration.The RF-603 connected to your camera "presses the shutter release button" through the bulb port (the cable), then the camera triggers the flash when its first curtain is down. The RF-603 has to be connected to the hot-shoe to sense that flash trigger and share the information with its buddies that are actually connected to the flash.______________________Edit on June 11th 2013In the original review I mentioned that remote-controlling the flash still required having the wire plugged in. Alana Right pointed out in the comments to my review for the RF-603 C1 that this was erroneous and indeed, I tried all the combinations of Canon body and flash I have at hand, and only one in one special case does seem to require the cable (most of the time). It turns out that you may want to make sure that your cold shoe does not short the contacts below the RF603 but otherwise the Canon bodies will trigger the remote flash even when not connected through the remote shutter cable.All in all unless of course you want to remote-control your camera, you don't need to plug in the remote control wire, essentially making the RF603 C1 and C3 entirely compatible as for flash triggering.This also means that unlike mentioned in my original review, the RF603 is compatible with using a cable for shutter release.Finally if you don't plan to use the cable, the RF-603 C3 is also compatible with other Canon bodies such as the G15 - I attach a picture above. It works just fine.Thanks again to Alana for taking the time to point this out.______________________Finally I have not tried mixing RF-603 C with RF-603 N. I would be surprised that they don't interact seamlessly as long as you attach a 603 N to any Nikon camera and a 603 C to any Canon camera, but it would be worth the experiment.