The paper presents the Universal Feynman Rules Output (UFO) format, a new model format for automated matrix-element generators. The UFO format is universal in that it is compatible with multiple generators and is flexible, modular, and agnostic of assumptions about the number of particles or color and Lorentz structures in interaction vertices. Unlike other formats that require parsing text files, UFO encodes model information into a Python module that can be easily linked to other codes. The paper describes an interface for the MATHEMATICA package FEYNRules that allows for automatic output of models in the UFO format. The UFO format is a Python-based format that allows for the representation of model information in an abstract form that can be accessed by other tools. The information on particles, parameters, and vertices is stored in a set of Python objects, each associated with a list of attributes related to their properties. This format allows for easier extension and modification of model information compared to traditional text-based formats. The UFO format is already used by the MADGRAPH version 5 and the GoSAM generators, and will be used in the future by HERWIG++. The UFO format is described in detail, including the definition of classes useful for designing model files. The format is based on the Python language, with all files having a .py extension. The UFO format allows for the representation of model information in an abstract form that can be accessed by other tools. The format is completely generic and does not make any a priori assumptions on the structures that can appear in a model. The information on the particles, parameters, and vertices of the model is stored in a set of Python objects, each of which is associated with a list of attributes related to their properties. The paper discusses the implementation of the UFO format, including the definition of the Particle, Parameter, Vertex, Coupling, Lorentz, and CouplingOrder classes. The UFO format allows for the representation of model information in an abstract form that can be accessed by other tools. The format is completely generic and does not make any a priori assumptions on the structures that can appear in a model. The information on the particles, parameters, and vertices of the model is stored in a set of Python objects, each of which is associated with a list of attributes related to their properties. The UFO format is designed to be flexible, modular, and agnostic of any assumptions about the structures that can appear in a model.The paper presents the Universal Feynman Rules Output (UFO) format, a new model format for automated matrix-element generators. The UFO format is universal in that it is compatible with multiple generators and is flexible, modular, and agnostic of assumptions about the number of particles or color and Lorentz structures in interaction vertices. Unlike other formats that require parsing text files, UFO encodes model information into a Python module that can be easily linked to other codes. The paper describes an interface for the MATHEMATICA package FEYNRules that allows for automatic output of models in the UFO format. The UFO format is a Python-based format that allows for the representation of model information in an abstract form that can be accessed by other tools. The information on particles, parameters, and vertices is stored in a set of Python objects, each associated with a list of attributes related to their properties. This format allows for easier extension and modification of model information compared to traditional text-based formats. The UFO format is already used by the MADGRAPH version 5 and the GoSAM generators, and will be used in the future by HERWIG++. The UFO format is described in detail, including the definition of classes useful for designing model files. The format is based on the Python language, with all files having a .py extension. The UFO format allows for the representation of model information in an abstract form that can be accessed by other tools. The format is completely generic and does not make any a priori assumptions on the structures that can appear in a model. The information on the particles, parameters, and vertices of the model is stored in a set of Python objects, each of which is associated with a list of attributes related to their properties. The paper discusses the implementation of the UFO format, including the definition of the Particle, Parameter, Vertex, Coupling, Lorentz, and CouplingOrder classes. The UFO format allows for the representation of model information in an abstract form that can be accessed by other tools. The format is completely generic and does not make any a priori assumptions on the structures that can appear in a model. The information on the particles, parameters, and vertices of the model is stored in a set of Python objects, each of which is associated with a list of attributes related to their properties. The UFO format is designed to be flexible, modular, and agnostic of any assumptions about the structures that can appear in a model.