Although by far the weakest of the known forces in nature, gravity pervades the universe and played an essential role in the evolution of the universe to its current state. Newton's law of universal gravitation and its elegant successor, Einstein's theory of general relativity, represent milestones in the history of science and provide the best descriptions we have of gravity. General relativity is founded on the principle of equivalence of gravity and acceleration; an inescapable consequence is that gravity governs the very geometry of space and time. This property of gravity distinguishes it from the other forces and makes attempts to unify all of the forces into a "theory of everything" exceedingly difficult. How well do we really understand gravity? Do the same laws of gravity apply to objects on the opposite sides of the universe as to particles in the microscopic quantum world? Current research is attempting to improve the precision to which the laws of gravity have been tested and to expand the realm over which tests of gravity have been made. Gravitational waves, predicted by general relativity, are expected to be observed in the near future. This unit will review what we know about gravity and describe many of the directions that research in gravitation is following.