Clinical research studies evaluating investigational medical products are systematically categorized into four main phases: Phase 1 trials (focusing primarily on safety, toxicity, pharmacokinetics, and dosage finding in small patient or volunteer groups), Phase 2 trials (expanding cohort sizes to evaluate preliminary efficacy and optimal dosing), Phase 3 trials (large-scale, randomized, controlled registrational studies confirming therapeutic benefits against standard of care), and Phase 4 trials (post-marketing surveillance studies monitoring long-term safety and real-world performance).