This chapter presents the linear dynamic analysis for complex structures, which is the most frequent case encountered in practice. For this situation, one of the most efficient modeling strategy is based on a formulation in the frequency domain (structural vibrations). There are many advantages to use a frequency domain formulation instead of a time domain formulation because the modeling can be adapted to the nature of the physical responses which are observed. This is the reason why the low-, the medium- and the high-frequency ranges are introduced. The different types of vibration responses of a linear dissipative complex structure lead us to define the frequency ranges of analysis.