In this chapter, the authors review approaches based on the measurement of the transmission matrix (TM). They discuss the potential of the photoacoustic TM for imaging deep inside the medium and explore the use of the TM for endoscopic imaging with multimode optical fiber. Although the output field pattern of an optical or acoustic wave transmitted through a disordered sample is uncorrelated with the incident wave, the transmitted field can be controlled by manipulating the incident wave. The TM was first introduced to characterize the scaling and statistics of electronic conductance. It has subsequently proved to be a powerful tool for describing the statistics of local and integrated transmission of classical waves. The challenges of making a perfect measurement of the TM and accounting for incompleteness in the measurement have stimulated the search for enhanced control of transmission and reflection and improved dynamic range in focusing.