This chapter discusses the factorization theorems for hard processes in Quantum Chromodynamics (QCD), which enable the application of perturbative calculations to important hadron processes. The basic problem addressed by these theorems is how to calculate high-energy cross sections, which are functions of three classes of variables: kinematic energy scales ($Q$), masses ($m$), and a renormalization scale ($\mu$). The renormalization scale is chosen to be large to utilize asymptotic freedom, but this leads to logarithms involving masses, indicating the importance of long-distance contributions. Factorization theorems separate these long-distance and short-distance behaviors systematically, allowing predictions for cross sections. The chapter covers three key processes: deeply inelastic scattering, single-particle inclusive annihilation, and the Drell-Yan process. For each process, the factorization theorem is derived and applied to extract parton distribution functions from experimental data. The hard scattering cross section, which is perturbatively calculable, is then used to predict the cross section for the original process. The intuitive basis for factorization theorems is provided through the parton model, where hadrons are described as composed of constituent partons (quarks and gluons) held together by interactions. The factorization theorem is shown to be a field-theoretic realization of this model, with parton distributions describing the internal structure of hadrons. The chapter also discusses the calculation of the hard scattering cross section, including the removal of infrared poles and the renormalization group equations that govern the behavior of the hard function under changes in the renormalization scale. The choice of renormalization scale is crucial to avoid large logarithms that could cancel the small coupling constant, and the chapter explains why $\mu \sim Q$ or $\mu \sim \sqrt{s}$ are preferred choices. Finally, the chapter defines the parton distribution functions in operator form, which are essential for both predicting and calculating the hard scattering cross section.This chapter discusses the factorization theorems for hard processes in Quantum Chromodynamics (QCD), which enable the application of perturbative calculations to important hadron processes. The basic problem addressed by these theorems is how to calculate high-energy cross sections, which are functions of three classes of variables: kinematic energy scales ($Q$), masses ($m$), and a renormalization scale ($\mu$). The renormalization scale is chosen to be large to utilize asymptotic freedom, but this leads to logarithms involving masses, indicating the importance of long-distance contributions. Factorization theorems separate these long-distance and short-distance behaviors systematically, allowing predictions for cross sections. The chapter covers three key processes: deeply inelastic scattering, single-particle inclusive annihilation, and the Drell-Yan process. For each process, the factorization theorem is derived and applied to extract parton distribution functions from experimental data. The hard scattering cross section, which is perturbatively calculable, is then used to predict the cross section for the original process. The intuitive basis for factorization theorems is provided through the parton model, where hadrons are described as composed of constituent partons (quarks and gluons) held together by interactions. The factorization theorem is shown to be a field-theoretic realization of this model, with parton distributions describing the internal structure of hadrons. The chapter also discusses the calculation of the hard scattering cross section, including the removal of infrared poles and the renormalization group equations that govern the behavior of the hard function under changes in the renormalization scale. The choice of renormalization scale is crucial to avoid large logarithms that could cancel the small coupling constant, and the chapter explains why $\mu \sim Q$ or $\mu \sim \sqrt{s}$ are preferred choices. Finally, the chapter defines the parton distribution functions in operator form, which are essential for both predicting and calculating the hard scattering cross section.