Ashoke Sen made a number of major original contributions to the subject of string theory, including his landmark paper on strong-weak coupling duality or S-duality, which was influential in changing the course of research in the field. He pioneered the study of unstable D-branes and made the famous Sen conjecture about open string tachyon condensation on such branes. His description of rolling tachyons has been influential in string cosmology. He has also co-authored many important papers on string field theory.