This chapter explores alternative relativistic field theories, focusing on scalar field models, higher-dimensional spaces, non-commutative geometry, non-linear theories, and complex scalar fields. It examines electrodynamics within these frameworks, considering extensions like Proca, Born-Infeld, Bopp-Podolsky, Chern-Simons, axions, and Kalb-Ramond field theories, leading to novel physical predictions. The chapter also discusses higher half-integer spinors, the Rarita-Schwinger equation, and alternative gravity theories such as f(R) theories, teleparallel gravity, Einstein-Cartan theory, Weyl’s conformal gravity, and varying G-cosmologies. Additionally, it covers classical relativistic strings and branes, their coupling to Kalb-Ramond fields, and introduces supersymmetric theories, emphasizing their mathematical foundations like Grassmann variables and superspace. The construction of superfields, differences in actions between bosonic and fermionic particles, and supersymmetric transformations are explored, highlighting their role in connecting bosonic and fermionic degrees of freedom. The chapter concludes with discussions on chiral superfields, particularly in models like the Wess-Zumino model, underscoring their significance in modern particle physics and quantum field theory.