The frequency shift andQchange of a microwave cavity caused by lossy dielectrics or plasma spheres or cylinders is calculated. The necessity for this analysis is prompted by the use of microwave cavities to measure the expansion of nearly spherical plasmas created by focused high‐power lasers. The conditions required for the perturbing volume to behave as if it were a perfect conductor are examined. It is found that much lower conductivities (or permittivities) are required when the perturbing volume is placed in a cavity region where electric energy is predominant.