This textbook is a self-contained introduction to tides that will be useful for courses on tides in oceans and coastal seas at an advanced undergraduate and postgraduate level, and will also serve as the go-to book for researchers and coastal engineers needing information about tides. The material covered includes: a derivation of the tide-generating potential; a systematic overview of the main lunar periodicities; an intuitive explanation of the origin of the main tidal constituents; basic wave models for tidal propagation (e.g. Kelvin waves, the Taylor problem); shallow-water constituents; co-oscillation and resonance; frictional and radiation damping; the vertical structure of tidal currents; and a separate chapter on internal tides, which deals with ocean stratification, propagation of internal tides (vertical modes and characteristics) and their generation. Exercises are provided in each chapter.