Frequency-dependent transmission line models can be developed in phase domain or in mode domain. Associated to these transmission line models, time domain or frequency domain can be used for electromagnetic transient analysis solutions. Because of these options, there are several models for transmission line representation in electromagnetic transient studies. One reason for this is the longitudinal parameter frequency dependence. This book presents a variety of models which serve to improve the frequency dependence representation. In some cases, phase-mode transformation is applied, considering the problem in mode domain, and searching for improving the frequency-dependent parameter representation. On the other hand, changes in the system structure as well as voltage and current surge waves are better modelled on the time domain.