The Alloy Theoretic Automated Toolkit (ATAT) simplifies the calculation of alloy thermodynamic properties from first-principles by implementing decision rules based on statistical analysis. It combines the Structure Inversion Method (SIM) with semi-grand-canonical Monte Carlo simulations to provide a high-level interface for researchers. ATAT consists of two main components: the MAPS code for constructing cluster expansions and the EMC2 code for performing Monte Carlo simulations. The MAPS code automates the construction of cluster expansions using first-principles calculations, while the EMC2 code runs Monte Carlo simulations to determine thermodynamic properties. The toolkit is designed to be user-friendly, allowing researchers to focus on higher-level aspects of first-principles thermodynamic calculations without the need for extensive expertise in coding and monitoring. ATAT is currently limited to binary systems but is being developed for multicomponent systems. Future developments will include the addition of vibrational and electronic contributions to the free energy calculations.The Alloy Theoretic Automated Toolkit (ATAT) simplifies the calculation of alloy thermodynamic properties from first-principles by implementing decision rules based on statistical analysis. It combines the Structure Inversion Method (SIM) with semi-grand-canonical Monte Carlo simulations to provide a high-level interface for researchers. ATAT consists of two main components: the MAPS code for constructing cluster expansions and the EMC2 code for performing Monte Carlo simulations. The MAPS code automates the construction of cluster expansions using first-principles calculations, while the EMC2 code runs Monte Carlo simulations to determine thermodynamic properties. The toolkit is designed to be user-friendly, allowing researchers to focus on higher-level aspects of first-principles thermodynamic calculations without the need for extensive expertise in coding and monitoring. ATAT is currently limited to binary systems but is being developed for multicomponent systems. Future developments will include the addition of vibrational and electronic contributions to the free energy calculations.