2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/2411I describe two difficulties with current research into the cosmic evolution of BL Lacertae Objects: (1). Possible sample incompleteness due to unrecognized (i.e., ``hidden'') BL Lacs; and (2). The absence of a viable physical model of the evolution, to which current and future observations can be compared.6 pages, invited talk at STScI ASP Conf. Series "Blazar Demographics" conf. proc. ed. P. Padovani & C.M. UrryAstrophysicstext