This chapter reviews the current state of knowledge on physical sources of image degradation within the medium and gives possible future research directions to further improve image quality in medical ultrasound imaging. It concentrates on the sources of image degradation arising from the heterogeneity of the scanned region. The chapter also focuses on conventional, i.e. single-sided ultrasound imaging of heterogeneous tissues, which relies on: the transmission of a set of pulsed ultrasound beams in the region of interest with an array transducer; the recording of ultrasound waves scattered by heterogeneities in the region of interest, typically with the same array transducer; and the use of an algorithm for image reconstruction that processes the recorded scattered echo signals. Until recently, real-time implementations of phase aberration correction on ultrasound scanners have achieved rather low imaging rates between 0.5 and 8.5 frames per second, because phase aberration correction requires advanced strategies and signal processing.