We review the literature of the past two decades on the possible evolution from the massive star Wolf-Rayet phase to the black hole phase, and on the masses of WR stars observed in binary systems. A list of 55 WR binaries is presented. The 19 WR binaries with RV solutions have WN star masses in the range 2-55 M. and WC star masses in the range 9-16 M.. We discuss four candidate WR+BH systems: WR 145a (Cyg X-3), WR 148 (HD 197406), BAT99-102 (R 140a2), and BAT99-116 (Mk 34).