The Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open-source, robust, and efficient libraries for computational stellar astrophysics. It includes a 1-D stellar evolution module, MESA star, which combines various numerical and physics modules to simulate a wide range of stellar evolution scenarios, from very low-mass to massive stars. MESA star solves the fully coupled structure and composition equations simultaneously, using adaptive mesh refinement and sophisticated time-step controls, and supports shared memory parallelism via OpenMP. It provides state-of-the-art modules for equation of state, opacity, nuclear reaction rates, element diffusion data, and atmosphere boundary conditions. Each module is a separate Fortran 95 library with a defined public interface, facilitating independent development and testing. MESA is used for a wide range of applications, including evolutionary tracks of very low-mass stars, brown dwarfs, and gas giant planets, the complete evolutionary track of a 1 M☉ star from the pre-main sequence to a cooling white dwarf, the Solar sound speed profile, the evolution of intermediate mass stars through the He-core burning phase and thermal pulses on the He-shell burning AGB phase, the interior structure of slowly pulsating B stars and Beta Cepheids, and the evolution of helium accretion onto a neutron star. MESA is open-source and can be downloaded from its project website. It is designed to be useful for a wide range of stellar physics applications and encourages contributions from the astrophysics community. MESA includes modules for numerical methods, microphysics, and macrophysics. The numerical methods module provides solvers for stiff and non-stiff systems of ordinary differential equations, as well as a Newton-Raphson solver for multidimensional, nonlinear root-finding. The microphysics modules provide physical properties of stellar matter, including the equation of state, opacities, and nuclear reaction networks. The macrophysics modules handle convection, diffusion, and atmosphere boundary conditions. The MESA modules are thread-safe, allowing applications to utilize multicore processors. The equation of state (EOS) is provided by the eos module, which uses density and temperature as independent variables. The EOS is based on the 2005 update of the OPAL EOS tables, with extensions to lower temperatures and densities using the SCVH tables. The opacity tables are constructed by combining radiative opacities with electron conduction opacities from Cassisi et al. (2007). The nuclear reaction rates are provided by the rates module, which includes thermonuclear reaction rates from Caughlan & Fowler (1988) and Angulo et al. (1999). The weaklib module calculates lepton captures and β-decay rates for high densities and temperatures. The screen module calculates electron screening factors for thermonuclear reactions. The net module implements nuclear reaction networks, and the jina module is an alternative nuclear network module that specializesThe Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open-source, robust, and efficient libraries for computational stellar astrophysics. It includes a 1-D stellar evolution module, MESA star, which combines various numerical and physics modules to simulate a wide range of stellar evolution scenarios, from very low-mass to massive stars. MESA star solves the fully coupled structure and composition equations simultaneously, using adaptive mesh refinement and sophisticated time-step controls, and supports shared memory parallelism via OpenMP. It provides state-of-the-art modules for equation of state, opacity, nuclear reaction rates, element diffusion data, and atmosphere boundary conditions. Each module is a separate Fortran 95 library with a defined public interface, facilitating independent development and testing. MESA is used for a wide range of applications, including evolutionary tracks of very low-mass stars, brown dwarfs, and gas giant planets, the complete evolutionary track of a 1 M☉ star from the pre-main sequence to a cooling white dwarf, the Solar sound speed profile, the evolution of intermediate mass stars through the He-core burning phase and thermal pulses on the He-shell burning AGB phase, the interior structure of slowly pulsating B stars and Beta Cepheids, and the evolution of helium accretion onto a neutron star. MESA is open-source and can be downloaded from its project website. It is designed to be useful for a wide range of stellar physics applications and encourages contributions from the astrophysics community. MESA includes modules for numerical methods, microphysics, and macrophysics. The numerical methods module provides solvers for stiff and non-stiff systems of ordinary differential equations, as well as a Newton-Raphson solver for multidimensional, nonlinear root-finding. The microphysics modules provide physical properties of stellar matter, including the equation of state, opacities, and nuclear reaction networks. The macrophysics modules handle convection, diffusion, and atmosphere boundary conditions. The MESA modules are thread-safe, allowing applications to utilize multicore processors. The equation of state (EOS) is provided by the eos module, which uses density and temperature as independent variables. The EOS is based on the 2005 update of the OPAL EOS tables, with extensions to lower temperatures and densities using the SCVH tables. The opacity tables are constructed by combining radiative opacities with electron conduction opacities from Cassisi et al. (2007). The nuclear reaction rates are provided by the rates module, which includes thermonuclear reaction rates from Caughlan & Fowler (1988) and Angulo et al. (1999). The weaklib module calculates lepton captures and β-decay rates for high densities and temperatures. The screen module calculates electron screening factors for thermonuclear reactions. The net module implements nuclear reaction networks, and the jina module is an alternative nuclear network module that specializes