We live in the age of technology where messages are transmitted in sequences of 0's and 1's through space. It is possible to make an error with noisy channels, so self-correcting codes become vital to eradicate all errors (as the number of errors is small). These self-correcting codes are widely used in the industry for a variety of applications including e-mail, telephone, remote sensing (e.g., photographs of Mars), amongst others.We will present some essentials of the theory in this book. Using linear algebra, we have the salient Hamming codes. The next level of coding theory is through the usage of ring theory, especially polynomials, rational functions and power series, to produce BCH codes, Reed-Solomon codes and the classical Goppa codes. Then we progress to the geometric Goppa code using Algebraic Geometry.