YBa2Cu3O7−&dgr;thin films patterned in the form of a meander line resonant structure, are used as a sensitive probe of vortex dynamics at microwave frequencies. The experiments enable highly sensitive measurements of the complex microwave surface impedanceZs(&ohgr;,T,H) of YBa2Cu3O7−&dgr;as a function of applied dc magnetic field, temperature, and frequency. TheZs(H,T) versusHdata yield direct experimental results for the vortex viscosity and pinning force constant, which are compared with microscopic theory. ©1995 American Institute of Physics.