The chapter introduces finite fields, so-called Galois fields, starting from prime fields to extension fields. The analogy of the extension of real numbers to complex ones is used to illustrate the vectorial and polynomial formulation of numbers in an extension field. Log and antilog tables making use of primitive polynomials, provide the basis for pseudo-noise (PN) sequences. Although not based on primitive polynomials, as an add-on, Barker sequences are also shortly described.