In two lectures we will summarize some useful properties of free fermions. In the first lecture we discuss some simple properties such as: canonical transformations, useful decompositions, diagonalization of free Hamiltonians, Wick's theorem and trace formulas. In the second lecture we will discuss Grassmann algebra and coherent basis, reduced density matrix and how to calculate entanglement entropy and formation probabilities. In both lectures we will keep the discussion as general as possible(independent of the dimension and the couplings). As an example we will normally concentrate on periodic one dimensional free fermions.