The PX Transmitter is the core of the best wireless flash system in the world, the PX System.
To take advantage of the eTTL/iTTL and High Speed Sync, all while avoiding the drawbacks of line of sight, short range and sunlight interference, you will need a PX Transmitter to control your remote flashes that have a PX Receiver attached. One transmitter will allow you to control as many flashes as you want.
Compatible Master Devices (Canon):Speedlites: 550EX, 580EX, 580EXII, 600EX-RT
Compatible Master Devices (Nikon):Speedlights: SB-700, SB-800, SB-900, SB-910 Commanders: SU800