This book provides an overview of the basic optics of the metalens, providing an introduction for students, scientists and engineers. The scope of this book is to introduce the optical fundamentals and basic optical design methods of a m etalens, where light propagation can be modelled by simple, and yet useful basic optics (e.g., the refraction law of metasurfaces, the wave front propagation, the metasurface phase profile, meta-atoms, etc.).