In this chapter, we discuss three examples of applications—communications channel modeling, estimation theory, and queueing analysis. For communications systems analyses, this chapter discusses the Linear Time-Invariant (LTI) system, Fourier transform, input-output relationship, and white noise process. For estimation theory, this chapter discusses linear minimum mean square error estimation and Kalman filter. For queueing theory, this chapter presents examples of queueing drawn from telephone call processing, cars queueing at a toll gate, etc., and discusses the basics of queueing theory. This chapter presents an analysis of the M/M/1 queue. This chapter also discusses the properties of Gaussian RVs.