Book excerpt: Active Prelude to Calculus is designed for college students who aspire to take calculus and who either need to take a course to prepare them for calculus or want to do some additional self-study. Many of the core topics of the course will be familiar to students who have completed high school. At the same time, we take a perspective on every topic that emphasizes how it is important in calculus. This text is written in the spirit of Active Calculus and is especially ideal for students who will eventually study calculus from that text. The reader will find that the text requires them to engage actively with the material, to view topics from multiple perspectives, and to develop deep conceptual understanding of ideas.Many courses at the high school and college level with titles such as "college algebra", "precalculus", and "trigonometry" serve other disciplines and courses other than calculus. As such, these prerequisite classes frequently contain wide-ranging material that, while mathematically interesting and important, isn't necessary for calculus. Perhaps because of these additional topics, certain ideas that are essential in calculus are under-emphasized or ignored. In Active Prelude to Calculus, one of our top goals is to keep the focus narrow on the following most important ideas. Those most important ideas include: functions as processes; average rate of change; a library of basic functions; families of functions that model important phenomena; the sine and cosine are circular functions; inverses of functions; exact values versus approximate ones; and long-term trends, unbounded behavior, and limits of functions. See more in the preface of the text at https: //activecalculus.org/prelude/preface-our-goals.html.The text is available in three different formats: HTML, PDF, and print, each of which is available via links on the landing page at https: //activecalculus.org/. The first two formats are free.