Recent observational successes are providing a new impetus to the study of binaries containing stellar-mass black holes. I will start with a very brief review of the current understanding of massive binary evolution leading to the formation of black-hole binaries. I will then focus on a few of the outstanding issues in interpreting both X-ray and gravitational-wave observations and propose pathways for improving our understanding of some challenging aspects of binary evolution. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 During his visit to Heidelberg, Professor Mandel will be available for meetings by arrangement with his hosts, Friedrich Roepke (roepke@uni-heidelberg.de) and Fabian Schneider (fabian.schneider@h-its.org)