Black-hole (BH) transients are a type of X-ray binary in which a stellar-mass BH accretes material from a low- mass star via an accretion disc. They spend most part of their lives in a dim, quiescent state, but display powerful outbursts when their luminosity increases by up to seven orders of magnitude in all wavelengths. X-ray and radio observations performed during the last couple of decades have provided a rich data base on BH transients. A strong coupling between the properties of the accretion flow and the presence of outflows, such as radio-jets and hot X-ray winds, has been found to be a fundamental characteristic of these systems, and, to a great extend, of X-ray binaries in general. In addition to this, and particularly since the spectacular case of the 2015 outburst of the BH transient V404 Cygni, cold optical accretion disc winds have been discovered in several systems, suggesting that these might also have a significant impact on the entire BH accretion process. I will review the state-of-the-art of this field, with emphasis on the studies that we are currently carrying out on these novel optical winds with a suite of the largest telescopes.