Poly(ADP-ribose) polymerase inhibitors (PARPi) are the first clinically approved agents designed to exploit synthetic lethality. Based on the recent approvals, PARPi became available for patients with metastatic castration-resistant prostate cancer (mCRPC). Unlike breast or ovarian cancers, where the approvals are limited to patients with BRCA1/2 alterations, in mCRPC PARPi are offered to patients with a broader spectrum of aberrations. A growing body of data indicates that alterations in specific homologous recombination repair (HRR) genes may confer different sensitivities to PARPi. Another challenging issue is the optimal testing methodology for identifying these aberrations. This comprehensive review presents the current place of PARPi in the treatment of mCRPC, provide biological rationale explaining mechanisms of their action and resistance, and discuss current clinical challenges along with avenues for future research.