Photon counting was introduced and developed during four decades relying on Photomultiplier Tubes (PMT), but interesting alternatives are nowadays provided by solid-state single-photon microdetectors. In particular, Silicon Single-Photon Avalanche-Diodes (SPAD) attain remarkable basic performance, such as high photon detection efficiency over a broad spectral range up to 1 micron wavelength, low dark counting rate and photon timing jitter of a few tens of picoseconds. In recent years SPADs have emerged from the laboratory research phase and they are now commercially available from various manufactures. However, PMTs have much wider sensitive area, which greatly simplifies the design of optical systems; they attain remarkable performance at high counting rate and can provide position-sensitive photon detection and imaging capability. In order to make SPADs more competitive in a broader range of applications it is necessary to face issues in semiconductor device technology. The present state of the art, the prospect and main issues will be discussed.