This chapter discusses the coupling of electromagnetic fields and waves into biological systems. It provides a common understanding of essential interactions and field coupling phenomena to facilitate better appreciation and understanding of their importance in research on biological effects and in scientific, industrial, and medical applications. Electromagnetic phenomena consist of electric and magnetic fields that change with space and time. Their spatial variation is dictated by the electromagnetic properties of the material medium, that is, electrical permittivity and magnetic permeability. Mobile counterions are associated with charges on cell membranes, and thus membrane capacitance dominates the behavior of the dielectric constant at low frequencies. This frequency dependence is a result of the dramatic change that membrane capacitance undergoes as the frequency increases at extremely low frequencies. Electromagnetic phenomena require that certain boundary conditions are satisfied at a boundary surface, where the tissue permittivity changes abruptly.