Congruence classes (or shorter congruences) are powerful mathematical tools, simplifying many calculations with integers. The arithmetic associated with congruences is called modular arithmetic. In this kind of arithmetic, we simplify calculations by replacing each integer with its remainder when divided by some fixed positive integer $n.$ The simplification effect of this is that the whole, infinite set of integers $\mathbb Z$ is replaced by a much smaller set $\mathbb Z_n$ which contains only a finite number $n$ of elements. We will see that we can add, subtract and multiply numbers in $\mathbb Z_n.$ Just as it is the case in $\mathbb Z$, there are some difficulties with the division. In this sense, $\mathbb Z_n$ inherits many properties of integers but, because it is finite, it is much easier to work with.