Two major complications plague ovulation induction and so-called controlled ovarian hyperstimulation (COH): ovarian hyperstimulation syndrome (OHSS) and multiple pregnancies. Both are regarded as serious complications in their own way. OHSS can be so serious that it may even be life threatening to the patient, and multiple pregnancy may also be life threatening but to the fetuses. Both are very largely preventable. Awareness of the possibilities that either may occur is the first step in their prevention. With this awareness, steps can be taken to minimize their occurrence. The main aim of this chapter and the next is to emphasize this awareness and to indicate steps that can be taken for prevention as well as management.