The fundamental principles of measurement uncertainty in building acoustics are found in the ISO Guide to the expression of Uncertainty in Measurement (GUM) and in ISO 12999-1 (2014). The sound pressure level in a room varies from one point to another, even in an ideal diffuse sound field. These variations are particularly large when the excitation is a pure tone or narrow band noise like a one-third octave band and, thus, some kind of spatial averaging is necessary in order to get a reasonably accurate measurement result. This chapter presents an overview of the theoretical background and methods for estimation of uncertainty in sound pressure level measurements and reverberation time measurements in a room. Below Schroeder's limiting frequency, the modal overlap index is small (less than 3) and the room modes may be too spread to fill the frequency band that is measured.