There is a growing demand for Systems-on-Chip, integrating microprocessors, on-chip memories, data converters and a variety of sensors, which are capable of reliable operation at high temperatures. For instance, modern aircraft industry demands microcontrollers and electric motors to operate at high temperatures, in order to replace present hydraulic structures. This thesis explains how to design digital SoC which are capable of reliable operation at high temperatures. The essential part of this thesis focuses on the design, implementation, fabrication and high-temperature measurements of on-chip Latch based SRAM, PowerPC e200 based microcontroller, digital temperature sensor and Flash A/D converter. Embedded on-chip SRAM modules are one of the most important components in the modern SoC. We analyze thermally-caused failures in the 6T SRAM cell and elaborate on its reliability. Further, we show that Latch based SRAM modules are preferable to 6T SRAM for reliable operation beyond 150C, by comparing two 1kB SRAM modules implemented in standard 0.18um SOI CMOS process. We demonstrate reliable SRAM operation at 275C (fmax = 10MHz, Ptot = 400mW), that is by far the highest reported operating temperature for digital on-chip SRAM module. Designing SoCs for reliable operation at elevated temperatures is a significant challenge, due to increased static leakage current, reduced carrier mobility, and increased electromigration. We propose to customize a PowerPC e200 based SoC by using a dynamically reconfigurable clock frequency, exhaustive clock gating, and electromigration-resistant power distribution network. We fabricated a 20x9mm2 chip implementing this design in 0.35um Bulk CMOS process. We present worldâ s first PowerPC based SoC for reliable operation at 225C (fmax = 30MHz, Ptot = 1.2W). This design outperforms previously reported PowerPC based SoCs, which are not operational at temperatures beyond 125C. The on-chip measurements of the p-n junction temperature allow reliability improvements for the SoC that operates at high temperatures. Low-resolution temperature measurements are efficiently used for adjusting the optimal operation frequency and supply voltage. We used the Time-to-Digital conversion technique to design a fully-digital temperature sensor. We designed and simulated a fully-digital 5bit temperature sensor for 10C resolution temperature measurements in between Tj,min = -45C and Tj,max = 125C. Further, using a single clock cycle Time-to-Digital conversion technique, we present a fully-digital 5bit Pulse based Flash ADC implemented in 0.18um Bulk CMOS process. Measurement results demonstrate the state-of-the-art power efficiency result of 450 fJ/conv (fmax = 83MHz, Ptot = 900uW).