Alkaloids are a heterogeneous group of organic compounds produced naturally and are characterized by the presence of at least one nitrogen atom in a heterocyclic ring, which is usually basic in nature. The therapeutic potential of acridone, quinoline, quinazoline of plant, marine, and animal origin is well documented. The unique properties of these natural products are thus exploited by scientific advances in modern cellular and molecular biology, analytical chemistry, and pharmacology. This chapter sheds light on occurrence and biosynthesis of these alkaloids with emphasis on their various pharmacological activities. It has been reported that quinoline alkaloids cinchonine; aaptolines A and B; thienoquinoline exhibits antitumor activity against different types of cancer cells. Apart from this, other pharmacological activities like antiviral activity of several acridones like glycosparvine, 9(10H)-Acridanone; dercitin; cyclodercitin etc. isolated from marine sources is increasingly recognized. Quinazoline alkaloids have also been exploited for their various pharmacological activities. The FDA has granted marketing authorizations for several quinazoline derivatives for clinical use as anticancer agents. These include quinazoline-containing products, such as gefitinib, erlotinib, lapatinib, afatinib, and vandetanib. In addition, antiinflammatory activities of versicomides E and F quinazoline isolated from various species of plants and fungus are well explored. This chapter also discusses the potential antihypertensive, anticonvulsant activity of these alkaloids which are beyond the scope of the chapter.