Three types of functions are used in conjunction with the measured activity integrals (Ã or ã) to estimate radiation doses. This chapter is devoted to the measurement and estimation of these functions. We will describe the point-source function (PSF), voxel source kernel (VSK), and the standard S matrix of the Medical Internal Radiation Dose (MIRD) approach. Each of these parameters has its applications in theoretical, animal, and clinical dose calculations. Additionally, some inert materials require dose estimates such as inside dosimeter volumes. While measurement was possible for early PSF representations, we will see that Monte Carlo (MC) methods have become standard for the calculation of all three formats of the various dose-estimating functions.