Nonlinear focusing and reflection of weak acoustic shocks are important problems both in medical ultrasound and in aeroacoustics. In aeroacoustics, studies of propagation and reflection from surfaces of high-amplitude N-wave are relevant to the sonic boom problem associated with jet noise and with the development of civilian supersonic aircraft. In medical applications, high-intensity focused ultrasonic acoustic fields are widely used for therapy and noninvasive surgery. In this context, investigation of nonlinear acoustic fields and their characterization are of great importance. In this thesis, the problem of characterization of high-amplitude N-wave generated in air by an electric spark was studied by two optical methods: a Schlieren method and a Mach-Zehnder interferometry method. Pressure waveforms were reconstructed either from the light intensity patterns in the recorded images or from the signal of photodiode using an Abel-type transform. The temporal resolution of reconstructed N-waves is six time better than that of the current stateof- the-art microphones. Thus, proposed optical methods are perspective tools for calibration of broadband acoustic microphones. Developed optical techniques were applied to study the irregular reflection of spark-generated N-wave from the plane rigid surface in air. The Schlieren optical system was used for visualization of reflection pattern while the Mach-Zehnder interferometry method was applied to reconstruct waveforms. It was shown that irregular reflection occurs in a dynamical way and the length of the Mach stem increases with the propagation distance. Moreover, the Mach stem formation was observed above the surface where the reflected front shock of the N-wave intersects with the incident rear shock. Characterization of nonlinear focused acoustic fields of medical devices is important to predict and to control induced biological effects in tissue. In the thesis, nonlinear propagation effects were analyzed for two modern medical devices: Duolith SD1 used in extracorporeal shock wave therapy and Philips C5-2 array probe used in preliminary experiments to move kidney stones by acoustic radiation force. A combined measurement and modeling approach was used for field characterization of devices: the boundary condition for the modeling was set to match low power measurements of the acoustic pressure field. In addition to characterization of real medical transducers, the theoretical investigation of nonlinear focusing of pulsed and periodic ultrasonic beams was performed for Gaussian and piston sources using the KZK equation. The saturation mechanisms were found different for cases of periodic and pulsed fields. The reflection from the rigid boundary is considered in the thesis as a process similar to focusing of axially symmetric beam since the normal derivative of the pressure on the axis of focused beam and at the rigid surface in reflection wave pattern is equal to zero. Under this light, the formation of spatial structures similar to Mach stem was observed at the focal area of medical transducers and was described in numerical simulations within the framework of KZK equation.