This paper explains the principles of operation of instruments which use ultrasonic waves to 'see with sound' enabling us to use ultrasonics for medical imaging, for nondestructive testing and materials evaluation, and for instrumentation in the process industries. The fundamentals of ultrasonics, necessary for the design of ultrasonic instruments and the interpretation of their measurements, are summarized, together with aspects of ultrasonic transducer design, Tables of wave velocities, acoustic impedance, transmission and reflection coefficients are included.