In this dissertation the transition rates, the ionization potential depression, and the optical spectra in dense plasmas are investigated within a quantum-statistical approach. The dynamical structure factors are introduced to account for the detailed spatial and temporal correlations and fluctuations of the plasma environment. The transition rates between Rydberg states are found to be more reasonably explained by a coherent quasi-classical wave packet description. As another important result, a general expression for the ionization potential depression in plasmas is obtained.