FeynRules is a Mathematica package that simplifies the implementation of new particle physics models. After the user defines the model's basic information, such as particles, parameters, and the Lagrangian, FeynRules automatically derives the Feynman rules and stores them in a generic format suitable for translation to various Feynman diagram calculation programs. The package includes interfaces for CalcHEP/CompHEP, FeynArts/FormCalc, MadGraph/MadEvent, and Sherpa, enabling a single model implementation to work across these programs. The package allows users to define models using a model file containing particle information, parameters, and the Lagrangian. The model file is processed to derive Feynman rules, which are then translated into the format required by the chosen Feynman diagram calculator. FeynRules supports both scalar and tensor parameters, with tensor parameters requiring additional index definitions. The package also includes functions for testing model properties and building models incrementally. The package is designed to be flexible, allowing users to choose their preferred Feynman diagram calculator and to extend its functionality. FeynRules is particularly useful for models involving effective theories with higher-dimensional operators and for developing new models based on the Standard Model. The package is implemented in Mathematica and is compatible with various platforms and operating systems. The running time depends on the complexity of the model, ranging from seconds for the Standard Model to minutes for more complex models. The package provides a general and theorist-friendly environment for model development and includes a comprehensive manual.FeynRules is a Mathematica package that simplifies the implementation of new particle physics models. After the user defines the model's basic information, such as particles, parameters, and the Lagrangian, FeynRules automatically derives the Feynman rules and stores them in a generic format suitable for translation to various Feynman diagram calculation programs. The package includes interfaces for CalcHEP/CompHEP, FeynArts/FormCalc, MadGraph/MadEvent, and Sherpa, enabling a single model implementation to work across these programs. The package allows users to define models using a model file containing particle information, parameters, and the Lagrangian. The model file is processed to derive Feynman rules, which are then translated into the format required by the chosen Feynman diagram calculator. FeynRules supports both scalar and tensor parameters, with tensor parameters requiring additional index definitions. The package also includes functions for testing model properties and building models incrementally. The package is designed to be flexible, allowing users to choose their preferred Feynman diagram calculator and to extend its functionality. FeynRules is particularly useful for models involving effective theories with higher-dimensional operators and for developing new models based on the Standard Model. The package is implemented in Mathematica and is compatible with various platforms and operating systems. The running time depends on the complexity of the model, ranging from seconds for the Standard Model to minutes for more complex models. The package provides a general and theorist-friendly environment for model development and includes a comprehensive manual.