The tides wash the margins of all the great oceans, regulate the lives of sea urchins and fishermen, power the great bore waves on rivers like the St. John, the Amazon, and the Severn. For most of us the tides are romantic, primeval, poetic. Standing on an ocean beach, we might be impressed by this tangible manifestation of the gravity of the distant rock we call the Moon, but few of us would be led to reflect on how fundamental the tides are to an understanding of gravity itself. But fundamental is the right word. In the modern view, the real signature of gravity, the part of gravity that can't be removed by going into free fall, is the tidal force, whose most spectacular effect on Earth is to raise the ocean tides. In this chapter we will examine this aspect of gravity, starting with the simplest effects first and working our way up to ocean tides and then to tides elsewhere in the Solar System and beyond. We will return again and again in later chapters to the fundamental role of tides. Indeed, many astronomical systems transmit tidal forces as signals right across the Universe, signals that we call gravitational waves.