Explore the fundamentals of fermionic strings in this 1 hour 28 minute lecture from the Topical Physics course (PHY453.5) delivered by Ashoke Sen at the International Centre for Theoretical Sciences. The lecture covers key aspects of supersymmetric string theories, including Free Heterotic String, Free Type II and Type I Superstring theories, interactions, picture changing operators, and amplitude computations. This advanced theoretical physics lecture requires prior knowledge of Quantum Field Theory, General Relativity, and String Theory 1. Part of a broader course that meets Tuesdays and Thursdays at 11:30 AM, this session helps students develop skills in computing the spectrum and interactions in supersymmetric string theories.