During the last 50 years electricity demand has significantly increased. Large amounts of electricity are transported from the generation to the consumption sites. Overhead lines or underground cables are used to transport the required energy. High voltage levels are used to reduce the transmission losses. Power transformers are responsible for linking the different voltage levels. In addition, power transformers must regulate the voltage of the electricity grid when needed. To achieve the regulation of the voltage, power transformers are equipped with on-load tap changers (OLTC). The winding of the transformer is equipped with tapped windings. The function of the tap changer is to adjust the output voltage of the power transformer by selecting the different tapped windings and without interrupting the load current. Different OLTC technologies are used in the market. The most commonly used technology of the last 50-60 years is the conventional oil type OLTC (OILTAP). In the last few years, time based maintenance is systematically being replaced by condition based maintenance. Condition based maintenance aims to reduce the costs that are associated with the maintenance of high voltage equipment and prevent possible failures. On-load tap changers constitute a vulnerable part of the power transformer and there is a number of power transformer failures attributed to OLTC failures. Diagnostic tools are used to assess the condition of tap changers. The condition assessment of the tap changer leads to a decision about the appropriate maintenance. This thesis investigates the possibility of a new diagnostic tool for on-load tap changers based on partial discharge (PD) measurements. The research project focuses on the major insulation defects that could hamper the operation of on-load tap changers. The differences between PD activity in star and delta connected tap changers are also investigated in this thesis. The conditions that are present in on-load tap changers are initially modeled by the use of small scale set ups. Signature PD patterns associated with the major insulation defects are obtained from the small scale set up. In addition, PD measurements are also performed in a small, single phase tap changer and in a large three phase, star connected tap changer. The main purpose of this research project is to investigate the applicability of PD measurements as a diagnostic tool to assess the condition of the insulation of on-load tap changers.