This paper introduces the Modules for Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks (MIST) project, a comprehensive set of stellar evolutionary tracks and isochrones computed using the state-of-the-art open-source 1D stellar evolution package MESA. The first part of the paper presents models with solar-scaled abundance ratios covering a wide range of ages (5 ≤ log(Age) [yr] ≤ 10.3), masses (0.1 ≤ M/M⊙ ≤ 300), and metallicities (−2.0 ≤ [Z/H] ≤ 0.5). These models are self-consistently and continuously evolved from the pre-main sequence (PMS) to the end of hydrogen burning, white dwarf cooling, or carbon burning, depending on the initial mass. The paper also includes a grid of models evolved from the PMS to the end of core helium burning for −4.0 ≤ [Z/H] < −2.0. Extensive comparisons with observational constraints and existing models are provided. The evolutionary tracks and isochrones are available for download from the project website. The paper is structured into several sections, covering the MESA code, adopted physics, solar model calibration, model outputs, and comparisons with existing databases and observational data. The MESA code is described, including its modular architecture, time step controls, adaptive mesh refinement, and parallelization. The adopted physics, such as abundances, microphysics, boundary conditions, diffusion, rotation, and mixing processes, are detailed. The solar model calibration and the construction of isochrones are discussed, along with the treatment of bolometric corrections. An overview of the properties of the models is provided, followed by comparisons with existing databases and observational data for low-mass and high-mass stars. The paper concludes with a discussion of caveats and future work.This paper introduces the Modules for Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks (MIST) project, a comprehensive set of stellar evolutionary tracks and isochrones computed using the state-of-the-art open-source 1D stellar evolution package MESA. The first part of the paper presents models with solar-scaled abundance ratios covering a wide range of ages (5 ≤ log(Age) [yr] ≤ 10.3), masses (0.1 ≤ M/M⊙ ≤ 300), and metallicities (−2.0 ≤ [Z/H] ≤ 0.5). These models are self-consistently and continuously evolved from the pre-main sequence (PMS) to the end of hydrogen burning, white dwarf cooling, or carbon burning, depending on the initial mass. The paper also includes a grid of models evolved from the PMS to the end of core helium burning for −4.0 ≤ [Z/H] < −2.0. Extensive comparisons with observational constraints and existing models are provided. The evolutionary tracks and isochrones are available for download from the project website. The paper is structured into several sections, covering the MESA code, adopted physics, solar model calibration, model outputs, and comparisons with existing databases and observational data. The MESA code is described, including its modular architecture, time step controls, adaptive mesh refinement, and parallelization. The adopted physics, such as abundances, microphysics, boundary conditions, diffusion, rotation, and mixing processes, are detailed. The solar model calibration and the construction of isochrones are discussed, along with the treatment of bolometric corrections. An overview of the properties of the models is provided, followed by comparisons with existing databases and observational data for low-mass and high-mass stars. The paper concludes with a discussion of caveats and future work.