A theoretical estimate has been made of the unloaded Q factor available from half and whole wavelength transmission-line resonators on an Al2O3 substrate. This takes ohmic losses into consideration as well as the radiation losses from various types of cavity termination. It is shown that optimisation of the Q factor of open-circuit terminated resonators demands a critical choice of substrate thickness and line characteristic impedance. Short-circuit terminated cavities generally have a substantially higher Q factor. A measurement technique has been developed, based on lightly coupled transmission probes. This has enabled practical measurements to be made of the unloaded Q factor of various microstrip cavities. Agreement with theory is satisfactory. Q factors up to 600 have been measured.