Dr Ramanathan's work concerns mainly three aspects of Number Theory: Study of arithmetical groups, Diophantine inequalities and Automorphic functions related to quadratic forms. The work on arithmetical groups, relates to the solution of some important problems concerning their finite generation, commensurability and maximality. The work on Diophantine inequalities is related to problems of Davenport on the density of values of quadratic and other arithmetical forms. His recent work on automorphic functions is related to Siegel formula and is a notable contribution to Analytic Number Theory.