The original concept of the echo mechanism in diagnostic medical ultrasound-of step impedance discontinuities-is investigated in terms of the numerical implications of more refined models. The effect of a discontinuity in the attenuation coefficients is found to have as much significance as the impedance discontinuity at low reflection interfaces. An approximate analytical model is developed for spatially varying changes in the impedance. Numerical calculations are presented for reflections from delta -function and Gaussian-envelope RF pulses for two different models: an impedance gradient, and a connective tissue layer. The models used are well documented in acoustics textbooks, but the numerical results for typical tissue parameters show that a wide variety of interface structures may give rise to reflected amplitudes in the same range as that determined by the step impedance model. It is suggested that experimental investigation of interface structures may increase one's understanding of the tissue-ultrasound interaction in diagnostic processes.