The rapid generation of molecular complexity from simple starting materials is of paramount importance in synthetic chemistry. The unique combination of high reactivity and high selectivity often associated with open-shell intermediates makes radical chemistry ideal for cascade reactions, in which simple substrates undergo a series of processes involving bond formation (and bond cleavage) to give complex, high-value products. Crucially, radical cascade reactions can greatly diminish the time, cost and amount of waste associated with complex target synthesis. Recent exciting advances in the field of radical chemistry initiated by single electron transfer (SET) have led to a considerable upward shift in our ability to design powerful new cascade reactions. This Review highlights recent advances in the development of radical cascades, triggered by SET processes, that deliver molecular constructs of importance in medicine and biology.