Basic principles of biomedical ultrasound imaging systems including conventional ultrasound and an emerging and promising ultrasound-mediated biophotonic imaging modality – photoacoustic imaging will be introduced. The covered topics are (1) overview of biomedical ultrasound with emerging technologies and trends, (2) acoustic wave propagation including elasticity imaging and B-mode contrast mechanism, (3) scattering, attenuation and speckle (ultrasound vs. optics), (4) tutorial on photoacoustic imaging, (5) real-time image formation, (6) diffraction and beam formation using arrays, and (7) color and spectral Doppler. In addition, learning by doing style is emphasized over which computer-homework assignments facilitate hand on experiences.