This book describes an alternative method of realizing accurate on-chip frequency references in standard CMOS processes. This method exploits the thermal-diffusivity of silicon, i.e. the rate at which heat diffuses through a silicon substrate. This is the first book describing the design of such electrothermal frequency references. It includes the necessary theory, supported by practical realizations that achieve inaccuracies as low as 0.1% and thus demonstrate the feasibility of this approach. The book also includes several circuit and system-level solutions to the precision circuit design challenges encountered during the design of such frequency references.