We electrically and optically tested both single pixels and complete arrays of Silicon Photomultipliers, from 5×5 to 64x64, fabricated by STMicroelectronics. Single cell devices operation was studied as a function of the temperature from -25°C to 65°C varying the voltage over breakdown, from 5% up to 20% of the breakdown voltage. Optical characterization was performed using a laser at 659 nm and opportunely chosen filters to vary the optical power. We determined the single pixel gain by using both the time resolved dark count signal and the current under controlled illumination. Typical gain values above 1×105 and above were obtained for operation times of 10 ns, while higher gains are obtained for longer integration times and lower photon flux.