This book is an introductory text on Theta function. It describes Classical Theta function, Ramanujan's Theta function and Cubic Theta function and Their developments and interconnection. This is designed for graduates and researchers as a start up material. The topics dealt with it includes Theta function as a solution of Heat conduction equation, elementary properties of Ramanujan's Theta function, One variable, Two variable, Three variable cubic theta functions and identities involving them.