The decay of uranium-238, uranium-235, thorium-232, and samarium-147 is accommodated through alpha-decays that produce helium-4 in geologic materials. Exploiting this parent-daughter system to measure time in rocks has roots that stretch back to Rutherford and the foundations of our understanding of nuclear decay, however, it is only recently that U-Th/He system has become widespread. The reason for the lag is that the accumulation of helium-4 within uranium and thorium bearing minerals is temperature dependent, meaning that at elevated temperatures characteristic of the crust, helium diffuses from mineral grains as rapidly as it is produced through alpha-decay. As rocks cool, the rate of helium diffusion drops exponentially. As a result, U-Th/He ages record the time since cooling, rather than the time since mineral growth. Uranium and thorium are present in trace amounts (ppm) in a number of different accessory mineral phases. Owning to differences in composition and crystallographic structure, the kinetics that govern helium diffusion vary between different minerals. The U-Th/He system is unique because it is sensitive to very low temperatures that, in the case of the mineral apatite (calcium phosphate), correspond several km’s depth within the crust.