Effect handlers have been gathering momentum as a mechanism for modular programming with user-defined effects. Effect handlers allow for non-local control flow mechanisms such as generators, async/await, lightweight threads and coroutines to be composably expressed. The Multicore OCaml project retrofits effect handlers to the OCaml programming language to serve as a modular basis of concurrent programming. In this talk, I will introduce effect handlers in OCaml, walk through several examples that illustrate their utility, describe the retrofitting challenges and how we overcome them without breaking existing OCaml code. Our implementation imposes negligible overhead on code that does not use effect handles and is efficient for code that does. Effect handlers are slated to land in OCaml after the addition of parallelism support.