Abstract: A new generation of power electronic systems with reduced size, losses and costs is emerging due to the rapid development of gallium nitride (GaN) transistors. These transistors are fabricated in the GaN-on-Si technology and have a lateral structure, which enables the monolithic integration with peripheral functions such as driver, sensing, protection, and control. In this manner, the first GaN power integrated circuits (ICs) with gate driver have successfully entered the market. This work extends the monolithic integration in the GaN technology by introducing and investigating building and function blocks for power ICs. A GaN power IC platform with devices and building blocks is investigated regarding its characteristics and its related design issues are analyzed. This leads to new design approaches and methods, which are presented in this thesis. Integration levels are introduced for the classification of GaN power ICs. Emphasis is placed on a schematic and layout design framework which provides circuit designers an effective way to implement custom power ICs. Two types of transistors and diodes as well as design approaches for layout structures with high current carrying capabilities are analyzed. Besides the investigation of active devices, also new approaches for integrated passives are introduced, such as dog bone layouts for high impedance resistors, p-GaN gate and stacked MIM capacitors as well as spiral inductors with high current carrying capabilities. Another focus is on the building blocks with digital and analog basic circuits. Area-efficient designs for logic gates with reduced static losses are presented. In addition, there are investigations into multi-stage comparators based on differential amplifiers and temperature-compensated voltage references. Based on this GaN power IC platform, two case studies are presented for a DC-DC synchronous buck converter and AC-DC active rectifier diode. The case studies on the one hand highlight the capability to implement complex functions and even systems consisting of devices and function blocks with their design approaches on a single chip and to use them advantageously for power electronics applications. Experimental measurements on implemented hardware prototypes, on the other hand, verify the advantages of highly-integrated GaN power ICs. The key findings of this work on GaN power integration are given. Beyond this comparisons of GaN and Si IC technologies as well as GaN power ICs are elaborated and perspectives, challenges and social benefits are discussed. This work contributes significantly to the progress and development of highly-integrated high-performance GaN ICs for power electronics