The key factors influencing microwave dielectric loss are examined. A comparison is made between single crystals and polycrystalline analogues. Measurements of the temperature dependence of microwave dielectric losses in various materials are reported, for temperatures between 20 and 300 K. Single crystal and polycrystalline TiO2, LaAlO3, MgO and Al2O3 are considered. The temperature dependence of dielectric losses of certain single crystals (MgO and Al2O3) are found to be in good agreement with the theory of intrinsic losses for temperatures above 100 K. At lower temperatures losses due to defects and grain boundaries dominate. The absolute value of the loss predicted by theory is considerably lower than measured values.