We give a rigorous, yet, we hope, readable, presentation of the Hopcroft-Tarjan linear algorithm for testing the planarity of a graph, using more modern principles and techniques for developing and presenting algorithms that have been developed in the past 10-12 years (their algorithm appeared in the early 1970's). Our algorithm not only tests planarity but also constructs a planar embedding, and in a fairly straightforward manner. The paper concludes with a short discussion of the advantages of our approach.