The study of knots in three-dimensional space is often done through their projections, called “knot diagrams”, onto different planes. A central question then is how to tell, given two such diagrams, whether they represent the same knot. This is typically answered through functions called “knot invariants”, the most classical among which is the Alexander polynomial. We will show three different ways of constructing it and mention some of its nice properties. Named after its creator, James Alexander, it first appeared in a paper in 1928 and we will discuss its relevance today, including generalizations to tangles and, time-permitting, categorification.