This lecture explores the impact of stochasticity in chemical reactions, comparing spatial deterministic models with stochastic models. It delves into the chemical master equation, transition probabilities, and the general solutions for irreversible 1st order reactions. The lecture also covers the derivation of probability density functions, the stochastic simulation algorithm, and the application of molecular particle models in enzyme kinetics. Furthermore, it discusses the effects of crowding on enzyme kinetics and the estimation of in vivo Michaelis-Menten parameters from initial-rate simulations.