We review the technique of 'specific heat spectroscopy,' of Birge and Nagel. The technique, also called the '3 omega method' in the literature, is nonadiabatic; it is based on thermal diffusion into a thick sample from a thin metallic him that serves simultaneously as heater and thermometer. Specific heat spectroscopy allows one to measure the dynamic specific heat of a liquid or solid over a frequency range exceeding 6 decades, and simultaneously the thermal conductivity over a more limited frequency range. Designed to study supercooled liquids near the glass transition, specific heat spectroscopy has also been used to study phase transitions and biological systems.