This chapter extends the path integral formulation of Quantum Field Theory to fermionic fields, which are represented by anti-commuting quantities. To achieve this, the chapter introduces the mathematics of Grassmann algebra, or anti-commuting c-numbers, establishing their unique rules for differentiation and integration. A key result derived is the formula for the Gaussian integral over Grassmann variables. This formalism is then applied to the free Dirac field to derive its generating functional and two-point function. Finally, the path integral is used to provide a rigorous, systematic derivation for the fundamental negative sign rule for every closed fermion loop in an interacting theory, tracing this rule back to the anti-commuting nature of the fermionic fields.