Meep is a free, open-source software package for simulating electromagnetic fields using the finite-difference time-domain (FDTD) method. It provides a flexible and powerful tool for researchers to model a wide range of electromagnetic phenomena, including nonlinear and dispersive materials, complex geometries, and various boundary conditions. The software is designed to be efficient, scalable, and user-friendly, with support for parallel computing and a flexible scripting interface. Meep is based on the Yee grid discretization, which allows for accurate simulation of Maxwell's equations by staggering electric and magnetic fields in space and time. The software supports a variety of coordinate systems, including Cartesian and cylindrical, and includes advanced features such as subpixel averaging, which helps to eliminate the first-order error typically associated with discrete grid-based simulations. It also includes a frequency-domain solver that leverages the time-domain code for faster convergence to steady-state solutions. The software is designed to be flexible and efficient, with a focus on performance and generality. It supports a wide range of materials, including anisotropic, nonlinear, and dispersive materials, and includes advanced signal processing techniques for analyzing resonant modes. Meep also includes a scripting interface that allows users to define complex simulations with minimal code, making it accessible to researchers with varying levels of expertise. Meep is particularly useful for simulating photonic crystals, waveguides, and other complex electromagnetic structures. It has been widely adopted in the research community, with over 100 journal publications citing the software and over 10,000 downloads. The software is available under the GNU GPL license and is implemented in C++, with support for additional free software packages such as GNU Guile, HDF5, and MPI. Meep is designed to be efficient and scalable, with support for distributed-memory parallelism and a flexible architecture that allows for easy adaptation to different computational environments. The software is particularly well-suited for simulations involving high-frequency electromagnetic waves, such as those found in integrated optics and photonic devices. Its ability to handle complex geometries and materials makes it a valuable tool for researchers in the field of computational electromagnetism.Meep is a free, open-source software package for simulating electromagnetic fields using the finite-difference time-domain (FDTD) method. It provides a flexible and powerful tool for researchers to model a wide range of electromagnetic phenomena, including nonlinear and dispersive materials, complex geometries, and various boundary conditions. The software is designed to be efficient, scalable, and user-friendly, with support for parallel computing and a flexible scripting interface. Meep is based on the Yee grid discretization, which allows for accurate simulation of Maxwell's equations by staggering electric and magnetic fields in space and time. The software supports a variety of coordinate systems, including Cartesian and cylindrical, and includes advanced features such as subpixel averaging, which helps to eliminate the first-order error typically associated with discrete grid-based simulations. It also includes a frequency-domain solver that leverages the time-domain code for faster convergence to steady-state solutions. The software is designed to be flexible and efficient, with a focus on performance and generality. It supports a wide range of materials, including anisotropic, nonlinear, and dispersive materials, and includes advanced signal processing techniques for analyzing resonant modes. Meep also includes a scripting interface that allows users to define complex simulations with minimal code, making it accessible to researchers with varying levels of expertise. Meep is particularly useful for simulating photonic crystals, waveguides, and other complex electromagnetic structures. It has been widely adopted in the research community, with over 100 journal publications citing the software and over 10,000 downloads. The software is available under the GNU GPL license and is implemented in C++, with support for additional free software packages such as GNU Guile, HDF5, and MPI. Meep is designed to be efficient and scalable, with support for distributed-memory parallelism and a flexible architecture that allows for easy adaptation to different computational environments. The software is particularly well-suited for simulations involving high-frequency electromagnetic waves, such as those found in integrated optics and photonic devices. Its ability to handle complex geometries and materials makes it a valuable tool for researchers in the field of computational electromagnetism.