The PicoHarp 300 is a high-end, easy-to-use, plug and play Time-Correlated Single Photon Counting (TCSPC) system. It is connected to a PC through an USB 2.0 high speed interface. A new design approach provides identical and synchronized but independent input channels. They can be used as detector inputs for coincidence correlation experiments or as a pair of start and stop inputs for TCSPC. It allows a forward start-stop operation even at full repetition rate of mode locked lasers with stable repetition rate up to 84 MHz. Experiments with low repetition rate benefit from the PicoHarp's multi-stop capability. The design allows high measurement rates up to 10 million counts/sec and provides a highly stable, crystal calibrated time resolution of 4 ps. This time resolution is well matched to the SPADs of the PDM series or micro-channel plate photomultiplier tubes (MCP). Overall IRF widths down to 50 ps can be achieved with pulsed diode lasers. With short pulsed lasers such as Ti:Sapphire lasers IRF widths as short as 30 ps can achieved. Both input channels are equipped with constant fraction discriminators (CFD), sensitive on the falling edge.