QuantumATK is an integrated platform for atomic-scale modeling developed by professional software engineers in collaboration with academic researchers. The platform offers a wide range of simulation engines, including density functional theory (DFT) and semi-empirical (SE) methods, to facilitate electronic-structure calculations and various physical simulations. DFT calculations can be performed using either a plane-wave basis or linear combination of atomic orbitals (LCAO), with pseudopotentials and exchange-correlation functionals. SE methods, based on tight-binding (TB) models, are also supported for efficient simulations of large systems. The platform includes advanced modules for electron transport, phonon properties, polarization, magnetic anisotropy, and quantum transport. It supports various boundary conditions and Poisson solvers, and can handle different types of atomistic configurations such as molecules, bulk materials, devices, and surfaces. QuantumATK provides a user-friendly graphical interface and scripting capabilities, making it accessible for both beginners and experts. The paper provides an overview of the platform, details on its implementation, and four application examples demonstrating the versatility of the simulation engines.QuantumATK is an integrated platform for atomic-scale modeling developed by professional software engineers in collaboration with academic researchers. The platform offers a wide range of simulation engines, including density functional theory (DFT) and semi-empirical (SE) methods, to facilitate electronic-structure calculations and various physical simulations. DFT calculations can be performed using either a plane-wave basis or linear combination of atomic orbitals (LCAO), with pseudopotentials and exchange-correlation functionals. SE methods, based on tight-binding (TB) models, are also supported for efficient simulations of large systems. The platform includes advanced modules for electron transport, phonon properties, polarization, magnetic anisotropy, and quantum transport. It supports various boundary conditions and Poisson solvers, and can handle different types of atomistic configurations such as molecules, bulk materials, devices, and surfaces. QuantumATK provides a user-friendly graphical interface and scripting capabilities, making it accessible for both beginners and experts. The paper provides an overview of the platform, details on its implementation, and four application examples demonstrating the versatility of the simulation engines.