RooFit is a C++ library for data modeling in the ROOT environment, providing a flexible framework for building complex fit models. It allows users to define probability density functions (PDFs) and perform fits, plotting, and Monte Carlo simulations. RooFit is used in high-energy physics analyses to model observable quantities in terms of physical parameters and detector effects. It supports both binned and unbinned likelihood fits, as well as chi-squared methods. The toolkit is designed to be efficient and user-friendly, with a focus on object-oriented mathematics and automated optimization. RooFit has evolved into an industrial-strength tool, capable of handling complex fits in experiments like BABAR, and is now available to all users via SourceForge. RooFit's core features include a natural vocabulary for building models, a data description language, support for fitting models to datasets, plotting tools, and event generation. It also provides computational efficiency, with models coded in RooFit as fast or faster than hand-coded models. The toolkit includes bookkeeping tools for configuration management and automation of routine tasks. RooFit uses an object-oriented approach, where mathematical concepts are represented as C++ objects. It supports the composition of complex models from elementary PDFs, and allows for the creation of new PDFs through mathematical transformations. The toolkit provides a wide range of PDFs, including Gaussian, exponential, and polynomial shapes, as well as physics-inspired and non-parametric PDFs. RooFit supports multi-dimensional models, which are natural extensions of one-dimensional models. It provides tools for fitting, plotting, and generating events for multi-dimensional datasets. The toolkit also includes advanced fitting options, such as the ability to define custom goodness-of-fit quantities and use optimization techniques like value caching, factorization, and parallelization. RooFit also provides data and project management tools, including automated PDF building and project configuration management. These tools help users manage large numbers of datasets and PDFs, and facilitate the creation of complex analyses. The toolkit has been widely adopted in the high-energy physics community, and is now available to all users via SourceForge.RooFit is a C++ library for data modeling in the ROOT environment, providing a flexible framework for building complex fit models. It allows users to define probability density functions (PDFs) and perform fits, plotting, and Monte Carlo simulations. RooFit is used in high-energy physics analyses to model observable quantities in terms of physical parameters and detector effects. It supports both binned and unbinned likelihood fits, as well as chi-squared methods. The toolkit is designed to be efficient and user-friendly, with a focus on object-oriented mathematics and automated optimization. RooFit has evolved into an industrial-strength tool, capable of handling complex fits in experiments like BABAR, and is now available to all users via SourceForge. RooFit's core features include a natural vocabulary for building models, a data description language, support for fitting models to datasets, plotting tools, and event generation. It also provides computational efficiency, with models coded in RooFit as fast or faster than hand-coded models. The toolkit includes bookkeeping tools for configuration management and automation of routine tasks. RooFit uses an object-oriented approach, where mathematical concepts are represented as C++ objects. It supports the composition of complex models from elementary PDFs, and allows for the creation of new PDFs through mathematical transformations. The toolkit provides a wide range of PDFs, including Gaussian, exponential, and polynomial shapes, as well as physics-inspired and non-parametric PDFs. RooFit supports multi-dimensional models, which are natural extensions of one-dimensional models. It provides tools for fitting, plotting, and generating events for multi-dimensional datasets. The toolkit also includes advanced fitting options, such as the ability to define custom goodness-of-fit quantities and use optimization techniques like value caching, factorization, and parallelization. RooFit also provides data and project management tools, including automated PDF building and project configuration management. These tools help users manage large numbers of datasets and PDFs, and facilitate the creation of complex analyses. The toolkit has been widely adopted in the high-energy physics community, and is now available to all users via SourceForge.