Plant-derived bioactive compounds represent a wide variety of naturally occurring chemical substances that have shown significant potential in preventing and curing human illnesses. This chapter presents a comprehensive overview of the structural classes and pharmacological actions of major phytochemicals, particularly phenolics, flavonoids, alkaloids, terpenoids, and organosulfur compounds. Examples such as curcumin, resveratrol, berberine, and allicin are discussed in terms of their mechanisms of action in combating oxidative stress, inflammation, microbial infections, neurodegenerative conditions, and cancer. In addition, this chapter highlights their methods of extraction and purification, recent advancements in analytical characterization techniques, and how they function on both cellular and molecular levels. The challenges of poor bioavailability, metabolic instability, and lack of standardization are critically examined, along with emerging strategies to enhance delivery and therapeutic efficacy, including nanoformulations. This integrative approach highlights the relevance of plant-derived bioactives as potential candidates for disease prevention and therapeutic intervention, joining the gap between traditional knowledge and recent drug discovery.