Medical ultrasound (US) is now widely used in medical anatomic and flow analysis and imaging. Diagnostic US encompasses ultrasonography and Doppler US. Like any other medical imaging tool, utility of US can be better realized when the performing technologist or physician is able to understand the principles behind its application. Efficient use of US also requires an understanding of physical variables which can be suitable, modified or optimized to produce a clear image of the anatomical or pathological structure. This brief narrative review summarizes the basic physics behind US. It highlights the necessary physical concepts such as wavelength, frequency and generation of US waves. These components are used as a frame of reference to explain important concepts such as descriptors of the US pulse, pulse-echo signal generation, attenuation, reflection, and scattering of US energy, speckle and echogenicity. It also informs the reader about the possible US and tissue interactions, use of US transducers and some recent developments of US techniques. The various artifacts that could be encountered during US study are also highlighted.