This chapter is intended to present the basic types of uncertainties found in many kinds of linear and nonlinear systems in order that in the upcoming chapters show how to design robust controllers for these kinds of systems. This chapter begins to explain the kinds of multiplicative and additive uncertainties along with their topology. As it is known, it is important to mention that in this chapter the kinds of uncertainties that are explained are of the parametric, nonparametric, and polytopic uncertainties that are commonly found in different kinds of complex dynamic systems. It is important to remark that these kinds of uncertainties are important to be mentioned considering that many kinds of physical systems like mechanical, electrical, and aeronautical for examples possess different types of uncertainties, taking into consideration that most of the physical systems possess unmodeled dynamics for the obvious reason that is very difficult to model real physical system in a mathematical representation. This chapter consists in describing the most commonly found uncertainties such as parametric and nonparametric uncertainties along with polytopic uncertainties and their topological representation. This chapter also includes the respective numerical examples to elucidate the respective uncertainty modeling.