The Alloy Theoretic Automated Toolkit (ATAT) is a software package that simplifies the calculation of alloy thermodynamic properties from first-principles. It implements the Structure Inversion Method (SIM) in combination with semi-grand-canonical Monte Carlo simulations. ATAT automates the process of constructing a cluster expansion, which is a mathematical representation of the alloy's energy based on atomic configurations. This cluster expansion is then used in Monte Carlo simulations to calculate thermodynamic properties and phase diagrams. The software provides a high-level interface, allowing researchers to focus on higher-level aspects of calculations without needing to monitor or manually process input/output files. ATAT consists of two main programs: the MAPS code for constructing the cluster expansion and the EMC2 code for Monte Carlo simulations. The MAPS code uses first-principles calculations to determine the effective cluster interactions (ECI), which define the Hamiltonian for Monte Carlo simulations. The EMC2 code then performs simulations to calculate thermodynamic properties. ATAT is modular, allowing individual components to be used as standalone tools. The software is designed to be user-friendly and accessible to a wide community of researchers. It enables the automated calculation of thermodynamic properties and phase diagrams, making first-principles calculations more efficient and practical. Future developments include expanding the toolkit to handle multicomponent systems and incorporating vibrational and electronic contributions to the free energy for better agreement with experimental data.The Alloy Theoretic Automated Toolkit (ATAT) is a software package that simplifies the calculation of alloy thermodynamic properties from first-principles. It implements the Structure Inversion Method (SIM) in combination with semi-grand-canonical Monte Carlo simulations. ATAT automates the process of constructing a cluster expansion, which is a mathematical representation of the alloy's energy based on atomic configurations. This cluster expansion is then used in Monte Carlo simulations to calculate thermodynamic properties and phase diagrams. The software provides a high-level interface, allowing researchers to focus on higher-level aspects of calculations without needing to monitor or manually process input/output files. ATAT consists of two main programs: the MAPS code for constructing the cluster expansion and the EMC2 code for Monte Carlo simulations. The MAPS code uses first-principles calculations to determine the effective cluster interactions (ECI), which define the Hamiltonian for Monte Carlo simulations. The EMC2 code then performs simulations to calculate thermodynamic properties. ATAT is modular, allowing individual components to be used as standalone tools. The software is designed to be user-friendly and accessible to a wide community of researchers. It enables the automated calculation of thermodynamic properties and phase diagrams, making first-principles calculations more efficient and practical. Future developments include expanding the toolkit to handle multicomponent systems and incorporating vibrational and electronic contributions to the free energy for better agreement with experimental data.