During his professional life, M. Wakatani made multiple contributions to the physics of magnetically confined plasmas. A major area of research was nonlinear MHD. He, directly and through many of his students, made multiple contributions to this area, particularly in the dynamics of resistive interchange and tearing instabilities in helical systems. The understanding of the sawtooth oscillations in stellarator/heliotron devices may be one of his main achievements. Prof. Wakatani spent a great deal of time on the study of helical systems and his work culminated in the physics design of the Heliotron J.