In this article the theoretical issues of high frequency electromagnetic fi eld propagation in complex living organism medium are studied in order to obtain a proper evaluation and scientifi c approach for non-thermal biological effects manifested at millimeter wave „in vivo” irradiation (30–300 GHz). Our main hypothesis starts from the following fact: living biological tissues are heterogeneous dielectrically structures and can be assimilated with a Fabry – Pérot resonator. Consequently, are studied the incident, respectively, refl ected and transmitted electromagnetic fi eld intensity of internal and refl ected wave. Taking into account the Maxwell equations, we obtain the relationships between electromagnetic fi eld energy densities. After applying a specifi c theoretical algorithm in which we consider that every of biological tissue layer is characterized by its own refraction and absorption coeffi cient, respectively by its own phase, we obtain the expression which characterizes the transmitted energy density after electromagnetic fi eld propagation through n layers. By this mathematical expression we can quantify the electromagnetic energy after propagation through a biological medium.