High energy density and power density are two important considerations in the design of a portable power source. High efficiency of energy conversion is one of the important factors in achieving high energy density. We report on the design, fabrication and characterization of a high efficiency and power density thermoelectric generator. Segmented thermoelectric elements with a high temperature-weighted average zT are used in the fabrication of the generator to achieve high energy conversion efficiency. A comprehensive model of the generator has been developed that includes temperature dependent thermoelectric properties for the segmented thermoelectric element design, and system level thermal and electrical losses. These models are used to guide the design of a high efficiency thermoelectric generator. A 14W size portable thermoelectric generator was built and its performance for converting thermal energy into electrical energy was characterized and compared to the model predictions. The magnitude of individual loss mechanisms and their effect on overall system efficiency are presented. The evolution of the device design and critical design parameters that have been incorporated or modified to minimize performance losses and improved device robustness are discussed. Important future design optimizations that can further reduce losses for improved performance were identified.