High-precision clocks play important roles in experiments to study gravitational phenomena. We illustrate these roles by discussing three proposed gravitational experiments involving clocks: a "null" gravitational redshift experiment using a hydrogen-maser clock and a super-conducting cavity-stabilized oscillator (SCSO) clock; a laboratory measurement of the Lense-Thirring effect using a toroidal microwave cavity; and a scheme for detecting gravitational waves from distant sources by means of doppler tracking of spacecraft, whose feasibility depends in part on future improvements in the stabilities of clocks.