This chapter describes room transfer functions in terms of poles and zeros. In general, a sound field in a room can be characterized by power response of a source located in the room. The resonant peaks of the responses due to the eigenfrequencies are observed even if the source locations were averaged throughout the room. This explains why eigenfrequencies are so significant in room acoustics. The power response could be controlled by using secondary sources closely located to the primary source, subject to the low modal overlap condition. In addition to the eigenfrequencies or poles, details of the phase characteristics of the transfer functions will be described, as the magnitude responses expressed by the modal density were in the previous chapters. Consequently, it will be shown how the propagation and reverberation phases are estimated according to the number of minimum and non-minimum-phase zeros distributed on the complex frequency plane instead of the eigenfrequencies.