The content of this short chapter is aimed at providing some degree of understanding concerning the methods by which measurements of the pumping speed and pump’s compression ratio should be carried out, and gaining insight into the difficulties we can expect. Both the constant volume and constant pressure methods are dealt with. For measurement of the compression ratio of (ultra-)high vacuum pumps a special double test volume separated by an aperture is the commonly accepted standard. It is essential for all measurements to be discussed, that they are carried out under operating conditions, i.e. a while of time must elapse in order that switch-on effects (warming-up, initial degassing) have been passed and the vacuum system has become in an equilibrium situation.