Transmission structures with uniform field distribution are used for testing emission and susceptibility of electronic devices. With the increase of frequency, the transmission structures get smaller in order to avoid resonances produced by the higher order modes. The resonances depend on the geometry of transmission structures. The improvement in the TEM-cell and GTEM-cell design can broaden the frequency range of use as well as enable larger objects to be tested. This work analyzes the basic transmission structures and numerical methods used for modeling the electromagnetic fields inside the TEM and GTEM-cell. Design of TEM-cell and GTEM-cell is explained. Extensive parameter measurements show the influence of absorbers and resistors in GTEM-cell design. Current distribution on septum depending on the resistor position is modeled. Frequencies of higher order modes are calculated. Measured EM field inside the cells confirms the numerical model results. The analysis given in the work should help EMC personnel and researchers in better understanding of TEM and GTEM-cells. The book can also be of use to teachers and Ph.D. students working in the area of electromagnetic compatibility.