Among the plants known for medicinal value, the members of genus Ocimum (family Lamiaceae) are very important for their therapeutic potentials. O.sanctum L., O.gratissimum L., O.canum Sims, O.basilicum L., O.kilimandscharicum Guerke and O.americanum L., are examples of some important species of genus Ocimum which grow in different parts of the world and are well known due to their excellent medicinal properties. Different parts of Tulsi and Tulsi Oil contain hundreds of beneficial compounds known as phyto-chemicals. Working together, these compounds possess strong antioxidant, antibacterial, antiviral, and immune-enhancing properties that promote general health and support the body's natural defense against stress and diseases. This book described the oil content, antioxidant properties, antimicrobial activities, immunomodulatory properties and valuable secondary metabolites. Further preliminary genetic analysis of four Ocimum species throws some light in distinguishing the superior variety in terms of biochemical activities and secondary metabolites. This monograph will popularize the use of Tulsi (Ocimum) for home remedy and scientific informatics to medicinal plant literature.