Do your students believe that the vertical line test is foolproof for determining whether a relationship is a function, or that every function can be modelled by an equation? How can you determine what they know or don?t know to move them forward in their thinking? This book details and explores best practices for teaching the essential ideas that students must grasp about functions, a topic that is challenging to learn and teach but which is critical to the development of mathematical understanding. The content covered includes the concept of a function, covariance, combining and transforming functions and graphs as representations of functions. The authors use classroom vignettes, and provide specific tasks and samples of student work to bring the topic to life. Easy to read, with clear and interesting explanations, this book will appeal to both experienced and novice practitioners.