CellSys is a modular, lattice-free simulation software for multi-cellular systems, enabling efficient 2D and 3D simulations of tissue growth and organization. It uses an agent-based model where cells are represented as isotropic, elastic, and adhesive spheres. The software includes modules for simulation, analysis, and visualization, with real-time 3D rendering and VRML 2.0 support. Parameters can be independently varied to support species-specific simulations and detailed analysis of growth dynamics and cellular interactions. CellSys is freely available for non-commercial use and can simulate growing monolayer cultures and avascular tumor spheroids. It has been used to model liver regeneration, with simulation results compared to experimental data. The software supports dynamic cell parameters and types, and integrates intracellular control modules by linking molecular concentrations to cell-level parameters. CellSys is implemented in C++ and is cross-platform, with a GUI based on OpenGL for 3D visualization. Core algorithms are parallelized using OpenMP, and the software is extensible, allowing future additions like modeling surrounding tissues or vascular networks. CellSys provides a robust framework for studying multi-cellular systems, offering insights into cellular and tissue-level phenomena through simulation and analysis.CellSys is a modular, lattice-free simulation software for multi-cellular systems, enabling efficient 2D and 3D simulations of tissue growth and organization. It uses an agent-based model where cells are represented as isotropic, elastic, and adhesive spheres. The software includes modules for simulation, analysis, and visualization, with real-time 3D rendering and VRML 2.0 support. Parameters can be independently varied to support species-specific simulations and detailed analysis of growth dynamics and cellular interactions. CellSys is freely available for non-commercial use and can simulate growing monolayer cultures and avascular tumor spheroids. It has been used to model liver regeneration, with simulation results compared to experimental data. The software supports dynamic cell parameters and types, and integrates intracellular control modules by linking molecular concentrations to cell-level parameters. CellSys is implemented in C++ and is cross-platform, with a GUI based on OpenGL for 3D visualization. Core algorithms are parallelized using OpenMP, and the software is extensible, allowing future additions like modeling surrounding tissues or vascular networks. CellSys provides a robust framework for studying multi-cellular systems, offering insights into cellular and tissue-level phenomena through simulation and analysis.