Drug development process flows seamlessly from Phase 0 to Phase 2 clinical trials, post which eligible drug candidates ultimately move into large-scale Phase 3 clinical trials. Phase 3 trials involve many more patients or volunteers than the earlier phases, and are considered the cornerstone in the human testing journey. The added benefit with Phase 3 trials is that there is also head-to-head comparison that is performed against existing standard therapy. Additionally, Phase 3 trials are also randomized in order to avoid bias. Blinding is also performed in many of the Phase 3 studies. Once a drug’s safety and efficacy are ascertained in large Phase 3 trials, the drug would be given marketing approval by the country’s regulatory body. Among the various phases of clinical trials, this phase is considered to be the most expensive as well as the most expansive. This chapter provides information on the various aspects involved in Phase 3 clinical trials.