BRL-CAD is a powerful cross-platform Open Source combinatorial Constructive Solid Geometry (CSG) solid modeling system that includes interactive 3D solid geometry editing, high-performance ray-tracing support for rendering and geometric analysis, network-distributed framebuffer support, image and signal-processing tools, path-tracing and photon mapping support for realistic image synthesis, a system performance analysis benchmark suite, an embedded scripting interface, and libraries for robust high-performance geometric representation and analysis. For more than 20 years, BRL-CAD has been the primary tri-service solid modeling CAD system used by the U.S. military to model weapons systems for vulnerability and lethality analyses. The solid modeling system is frequently used in a wide range of military, academic, and industrial applications including in the design and analysis of vehicles, mechanical parts, and architecture. The package has also been used in radiation dose planning, medical visualization, computer graphics education, CSG concepts and modeling education, and system performance benchmark testing among other purposes. BRL-CAD supports a great variety of geometric representations including an extensive set of traditional CSG primitive implicit solids such as boxes, ellipsoids, cones, and tori, as well as explicit solids made from closed collections of Uniform B-Spline Surfaces, Non-Uniform Rational B-Spline (NURBS) surfaces, n-Manifold Geometry (NMG), and purely faceted mesh geometry. All geometric objects may be combined using boolean set-theoretic CSG operations including union, intersection, and difference.