CHARMM is a versatile and widely used molecular simulation program developed over the past three decades, focusing on biological molecules such as proteins, lipids, and nucleic acids. It provides a comprehensive suite of computational tools for studying these systems, including conformational sampling, free energy estimation, molecular dynamics, and analysis techniques. CHARMM is applicable to a wide range of many-particle systems and can use various energy functions, from quantum mechanical-molecular mechanical force fields to all-atom classical potential energy functions with explicit solvent. The program has been ported to numerous platforms and is continuously updated to incorporate new methods and improve its flexibility and efficiency. The paper provides an overview of CHARMM as it exists today, emphasizing developments since its original publication in 1983. It also reviews the program's origin, management, distribution, and future directions. CHARMM is a general-purpose molecular simulation and modeling program that uses classical and quantum mechanical energy functions for various molecular systems. It has been used to study a wide range of biomolecules and has been extended to support new methods and models. The program includes a scripting language for controlling simulations and has been used to develop new force fields and models. The paper also discusses the potential energy function, including the force fields used in CHARMM, and recent developments such as the introduction of the CMAP correction term for the protein backbone and the treatment of induced polarization. The paper highlights the importance of CHARMM in biomolecular simulations and its continued development to improve accuracy and efficiency.CHARMM is a versatile and widely used molecular simulation program developed over the past three decades, focusing on biological molecules such as proteins, lipids, and nucleic acids. It provides a comprehensive suite of computational tools for studying these systems, including conformational sampling, free energy estimation, molecular dynamics, and analysis techniques. CHARMM is applicable to a wide range of many-particle systems and can use various energy functions, from quantum mechanical-molecular mechanical force fields to all-atom classical potential energy functions with explicit solvent. The program has been ported to numerous platforms and is continuously updated to incorporate new methods and improve its flexibility and efficiency. The paper provides an overview of CHARMM as it exists today, emphasizing developments since its original publication in 1983. It also reviews the program's origin, management, distribution, and future directions. CHARMM is a general-purpose molecular simulation and modeling program that uses classical and quantum mechanical energy functions for various molecular systems. It has been used to study a wide range of biomolecules and has been extended to support new methods and models. The program includes a scripting language for controlling simulations and has been used to develop new force fields and models. The paper also discusses the potential energy function, including the force fields used in CHARMM, and recent developments such as the introduction of the CMAP correction term for the protein backbone and the treatment of induced polarization. The paper highlights the importance of CHARMM in biomolecular simulations and its continued development to improve accuracy and efficiency.