The CGNS (CFD General Notation System) file format is a standardized system designed to facilitate the storage and retrieval of computational fluid dynamics (CFD) data. It was developed to address the lack of a common standard for exchanging data between different CFD software packages, including grid generators, solvers, and post-processors. CGNS is built upon the HDF5 (Hierarchical Data Format version 5) standard, which provides a robust and scalable framework for handling large-scale scientific datasets. The format supports a wide range of data types, including structured and unstructured grids, flow solutions, boundary conditions, and connectivity information. Because it is platform-independent and open-source, CGNS has become a widely adopted standard in the aerospace and automotive industries, as well as in academic research. It allows engineers and scientists to move data seamlessly between different tools in their simulation workflow, ensuring data integrity and reducing the need for custom translation scripts. The system includes a formal data model, a library of C and Fortran routines known as the Mid-Level Library, and a physical storage layer, making it highly versatile for complex engineering simulations and long-term data archiving.