By using the multi-reflection theory of the SH-wave with a special emphasis on the effects of soil profiles and nonlinear soil properties during strong earthquakes, this chapter discusses the site amplifications in horizontally-layered soil systems. In order to measure the site amplification, two earthquake observation systems may be available; a horizontal array system at different surface geologies, and a vertical array system consisting of the ground surface and downhole depths of the different geologies in the same place. The site amplification between ground surface and base can be defined in various ways. Microtremors are ambient ground vibrations with unnoticeably small amplitudes caused by natural phenomena such as ocean waves and winds and also by human activities such as traffics and machines. Unlike the earthquake waves, however, microtremors consist of various source-specific body and surface waves which are sometimes very localized.