Platinum agents have been used in the treatment of ovarian cancer, and recently, PARP inhibitors have also been increasingly utilized, especially in cases with mutations in BRCA1 or BRCA2, or those with homologous recombination deficiencies. Acquired resistance to platinum and PARP inhibitors has become a clinically important issue. Various mechanisms of resistance have been proposed, among which “functional restoration through secondary mutations in BRCA1 or BRCA2 (reversion mutation)” holds clinical importance. This chapter provides an overview of recent insights into the molecular mechanisms of PARP inhibitor resistance, with a focus on secondary mutations in BRCA1 or BRCA2.