Many physical phenomena can be modeled using Monte Carlo simulation (MCS) because it is a powerful tool to study thermodynamic properties. MCS can be used to simulate interactions between several particles or bodies in the presence of local or external fields. The main idea is to create a high number of different random configurations; statistics can be taken according to appropriate algorithms (Metropolis algorithm). In this chapter, we present basic techniques of MCS as the choice of potential, reaction rates, simulation cell, random configurations, and algorithms. We present some principal ideas of MCS used to study particle-particle collisions in the gas and in plasmas. Other MCS techniques are presented briefly. A numerical application is presented for collisions in gas phase during thin film deposition by plasma-enhanced chemical vapor deposition (PECVD) processes. Parameters and results of the simulation are studied according to a chosen reactor and mixture.