During the last years a mayor effort has been carried out both, in defining key parameters to quantify the quality of a site for astronomical observations, and to design reliable techniques and tools to compare different sites. Here, we will revise some of the parameters relevant for astronomical site evaluation, and we will also brief on the instruments currently available for their measurements. The Observatories at the Canaries, Observatorio del Roque de los Muchachos (ORM) and Observatorio del Teide (OT) have been used as test bench for the development of new techniques and tools for more than three decades. Results on statistical measurements and techniques, emphasizing the most recent ones in the framework of the FP6 site selection program at the Canarian Observatories are given.