The nonlinear properties of gaseous contrast agents can be used to create new ultrasound imaging methods, such as harmonic and subharmonic imaging. Those methods are capable of separating the echoes from the contrast bubbles in blood and those from surrounding tissue. Harmonic images are produced by transmitting sound pulses at one frequency, but receiving echoes at double that frequency, while subharmonic images receive echoes at half that frequency. Experiments revealed that the subharmonic component of echoes from some contrast agents, at relatively high acoustic pressure, was greater than the second-harmonic component. Since harmonic imaging suffers from a reduction in image contrast, due to second-harmonic generation during ultrasound propagation, as well as second-harmonic leakage from the transmitted signal, this finding makes subharmonic imaging an attractive alternative to harmonic imaging. Harmonic response measurements with a pulse--echo arrangement and subharmonic images obtained with a modified scanner will be presented. The acoustic pressure threshold for subharmonic generation was found to be quite different for contrast agents with different compositions. The optimal transmitted ultrasound pulses and the reception strategy for subharmonic imaging will also be discussed.