Alkaloids are a class of naturally occurring organic compounds containing basic nitrogen atoms, primarily found in plants, fungi, and marine organisms. With their diverse chemical structures and significant biological activities, alkaloids have been of great interest in natural products chemistry for centuries. This review discusses the chemical properties, biosynthesis, classification, and biological activities of alkaloids. Emphasis is placed on their role in drug discovery and development, particularly in cancer treatment, pain management, and antimicrobial therapies. The potential of alkaloids as lead compounds for developing new therapeutic agents is explored, along with the challenges in their extraction and synthetic modification.