•OpenMC is an open source Monte Carlo particle transport code.•Solid geometry and continuous-energy physics allow high-fidelity simulations.•Development has focused on high performance and modern I/O techniques.•OpenMC is capable of scaling up to hundreds of thousands of processors.•Other features include plotting, CMFD acceleration, and variance reduction. This paper gives an overview of OpenMC, an open source Monte Carlo particle transport code recently developed at the Massachusetts Institute of Technology. OpenMC uses continuous-energy cross sections and a constructive solid geometry representation, enabling high-fidelity modeling of nuclear reactors and other systems. Modern, portable input/output file formats are used in OpenMC: XML for input, and HDF5 for output. High performance parallel algorithms in OpenMC have demonstrated near-linear scaling to over 100,000 processors on modern supercomputers. Other topics discussed in this paper include plotting, CMFD acceleration, variance reduction, eigenvalue calculations, and software development processes.