Internal dosimetrists are concerned with the development, selection, and calibration of biokinetic models to calculate radiation doses from incorporated radionuclides. This is accomplished using measurements of radionuclides in organs, tissues, and excreta, i.e., bioassay measurements. Each bioassay measurement has a corresponding likelihood function, which represents the relative likelihood of different biokinetic model parameters resulting in the measurement value. In order for a bioassay measurement to be interpreted properly, the correct likelihood function must be determined. Failing to use the correct likelihood function for each bioassay measurement results in improperly weighting certain measurements over other measurements, which in turn leads to incorrect dose estimates. This paper describes the correct likelihood functions to use for a wide variety of bioassay measurements, as well as a description of how to use them. These likelihood functions represent the vast majority of those likely to be needed for interpreting bioassay measurements. Therefore, this paper may serve as a tool kit that can be used by academic and occupational internal dosimetrists.