Kraft was a member of the Studiorum Novi Testamenti Societas (SNTS), the International Organization for Septuagint and Cognate Studies (IOSCS) and the American Society of Papyrologists.

Kraft was a co-founder and coordinator of the Philadelphia Seminar on Christian Origins (PSCO), which he established upon coming to Penn in 1963.

In his scholarship, Kraft made contributions to study of the Apostolic Fathers, particularly the Epistle of Barnabas, as well as the history and literatures of Judaism in the Greco-Roman Period, particularly the Septuagint (LXX) and Old Greek (OG) translations and their textual histories, and also Philo of Alexandria and his world.

Under Kraft's leadership and co-editorship (with G. Krodel), the Philadelphia Seminar on Christian Origins produced an English translation, with updated annotation, of Walter Bauer's classic Rechtgläubigkeit und Ketzerei im ältesten Christentum (published as Orthodoxy and Heresy in Earliest Christianity by Fortress Press, Philadelphia, 1971).

His publications include the following: Kraft preferred to publish his works electronically, and many of his writings may be found on his homepage.