Single photon avalanche diodes (SPADs) are avalanche photodiodes specifically designed for reverse bias operation above the breakdown voltage and used for detecting single optical photons. A new silicon epitaxial device structure was designed to give improved timing performance with respect to previous SPADs. Extensive tests were carried out in order to establish the timing resolution of the device in time correlated photon counting (TCPC). The timing resolution of the SPAD in terms of its full-width at half-maximum (FWHM) contribution to the overall instrumental response width is 20 ps with the detector cooled to -65 °C, and 28 ps at room temperature. This is the highest resolution so far reported for solid-state single-photon detectors. In vacuum tubes, comparable results are obtained only with special microchannel-plate photomultipliers (MCP-PMT). Results from time-resolved photoluminescence measurements in GaAs demonstrate the power of the TCPC technique when used with the new SPAD detector. With the excellent timing resolution of the SPAD and the well-known advantages of TCPC systems (high sensitivity, linearity, etc.), various applications are foreseen in areas so far dominated by streak cameras.