This thesis presents the modelling, design, and implementation of fully-integrated dc-dc converters. Since simple and accurate integrated inductor models for dc-dc converters are not currently available, the main objective of this thesis is to develop new models and compare the analysis, simulation, and measurement results of a proposed design. To achieve that, the circuit theory of both step-up and step-down converters is first presented in detail. The existing models, developments, and techniques in implementing fully integrated dc-dc converters are also highlighted. Complete mathematical models, which take the relevant losses into consideration for both the step-up and step-down converters, are developed next. A dedicated model for the integrated inductor that is tailored for power converters along with its parasitic resistance is also developed and presented. The proposed inductor model further optimizes the geometry without increasing the area; this consequently maximizes the performance of the inductor. Finally, an overall optimization algorithm combining both the converter and the inductor models is proposed and discussed in detail. Several converter topologies with different inductor geometries are designed and compared. Two step-up and two step-down converters are designed, simulated and compared. Each set contained a circuit with a square inductor and a circuit with the proposed inductor geometry. A fast pulse-width-modulated control system is designed to regulate the converters. Simulation results are presented, demonstrating the improvements in performance and conversion efficiency of the converter based on the proposed inductor geometry over that of the square-inductor based converter. A good agreement between the analytical calculations and the simulated results is reported. A prototype containing the four converters was fabricated in a 65-nm digital CMOS process. The feasibility of fabricating efficient fully integrated dc-dc converters is demonstrated and recommendations for future research are given.