In making the statement quoted opposite about the elastic properties of objects, Robert Hooke rather overstated the case. The restoring forces in any actual physical system are only approximately linear functions of displacement. This chapter presents the properties of matter that control the frequency of a mass-spring type of system. If a floating object is slightly depressed or raised from its normal position of equilibrium, there is called into play a restoring force equal to the increase or decrease in the weight of liquid displaced by the object, and periodic motion ensues. The chapter considers a number of examples of such motions, with particular emphasis on the way in which physicists can relate the kinematic features of the motion to properties that can often be found by purely static measurement. It looks at the system that forms a prototype for so many oscillatory problems—a mass undergoing one-dimensional oscillations under the type of restoring force postulated by Hooke.