During microwave and high frequency heating, many variables in the food affect the heating performance. Among the most significant is the permittivity of the food, which describes how a material interacts with microwaves. The permittivity is a complex quantity which has two separate parts: the real permittivity, epsilon＇ and the imaginary component, epsilon＇＇, the dielectric loss factor The permittivity describes a material＇s ability to absorb, transmit and reflect electromagnetic energy. Many measurement techniques for measuring permittivity are available; their advantages and limitations determine the choice of the measuring system. For water and some simple mixtures, models exist for predicting dielectric properties. These properties vary with the composition of a substance, and with the temperature and frequency. Water is the major constituent of most foods; thus its dielectric properties also determine to a great extent, the dielectric properties of the food. Therefore, these properties can be used in measuring water content. Knowledge of these properties is useful in constructing ovens, in selecting packaging materials, and in developing foods.