With the deregulation of the power industry in many countries, the traditionally vertically integrated power systems have been experiencing dramatic changes leading to competitive electricity markets. This chapter describes a theoretical study on probabilistic small signal stability assessment techniques so that the study results can be used to help RTOs and ISOs perform planning studies under the open access environment. By using the methodology developed in this chapter, system planners can model the system uncertainties when performing small signal stability analysis. The chapter presents the detailed power system modeling methods to include the major system components such as generator, excitation controller, system constraints, and other dynamics as differential equations. It derives the essential stator equations and network admittance matrices (along with their reduction) as algebraic equations. The chapter also describes both analytical and numerical approaches to compute the sensitivity of the system state matrix to certain parametric variations including control system parameters and system loads.