[1] His early work involved the use of Superspace to treat supersymmetric theories, including supergravity.
Grisaru, and M. Rocek he discovered methods for both deriving classical actions, and performing Feynman graph calculations more simply than those in Non-Supersymmetric theories.
The first supersymmetric nonrenormalization theorem was introduced by Grisaru, Siegel and Rocek in their 1979 paper "Improved methods for Supergraphs", which has close to 700 citations.
The textbook is also notable in that it is completely an "electronic book" and can be downloaded for free from Siegel's website.
[5] He served as a co-editor of the Concise Encyclopedia of Supersymmetry and Noncommutative Structures in Mathematics and Physics.