The number of BL Lacs discovered in the last 20 years is very small (≃ 200) if compared to that of quasars. Owing to their featureless optical spectrum, BL Lacs have been discovered mainly in radio (RBL) and X-ray surveys (XBL). The limited statistics available prevents a detailed study of the properties of BL Lacs and, in general, all the conclusions based on the present data sets are affected by large uncertainties. Yet, current results are intriguing, for instance RBL and XBL are found to have a cosmological evolution that differs not only in magnitude but most of all in sign (e.g. Stickel et al., 1991 and Wolter et al., 1994 and reference therein). Therefore, we have initiated a project aimed at discovering a significant number of new BL Lac objects (≃ 100), exploiting the fact that they are both radio and X-ray emitters and in particular the fact that they occupy a well defined region in the αox –αro plane. The method we use rests on the expertise grown during the construction of the EMSS sample (Gioia et al. 1990, Stocke et al. 1991) and is described in detail in Wolter et al. (1995).