OpenStudio is an open-source SDK (software development kit) for building energy simulation. OpenStudio greatly reduces the cost of developing end-use applications and services that exploit energy simulation by providing a large library of common helper functions for creating, querying, and transforming energy models, running simulations, and working with results. OpenStudio updates are released quarterly—the most recent version update, 1.9.0, was released 9/30/2015. A significant—and thus far unique—feature of OpenStudio is that its API (application programming interface) is accessible via a variety of scripting languages including Ruby and Python and OpenStudio itself can execute scripts written in these languages. The scripting facility allows OpenStudio to be customized and extended in a flexible way. The most common use of scripting is to automate energy conservation measures that can be applied to existing models, and as a result scripts are commonly referred to as “measures”. However, scripts/measures can be—and are being—used to do a variety of things. OpenStudio measures—as well as static simulation content like HVAC component descriptions, construction descriptions, weather files, standard schedules, etc.—are stored in the open, dynamic BCL (Building Component Library) database (http://bcl.nrel.gov/). The BCL allows measures and content to be vetted by and shared with a group or with the larger community. OpenStudio also includes a suite of graphical applications that demonstrate the power of the SDK. These include a plug-in for Trimble SketchUp for creating 3D geometry; a companion graphical application for creating other aspects of an energy model such as constructions, schedules, and HVAC systems; and a parametric analysis tool for applying measures to models. The OpenStudio SDK is also available as an AMI (Amazon Machine Image) which allows users to run large simulation studies on Amazon’s EC2 (Elastic Compute Cloud) service. Simulation studies are specified using an easy-to-use Excel interface and extensively use the concept of measures. Originally developed for EnergyPlus, OpenStudio now also supports the energy simulation engine ESP-r, CEN/ISO 13790 annual and hourly energy calculations, the lighting engine Radiance, the airflow engine CONTAM, and the code compliance engine CBECC-Com. Learn more at http://openstudio.net.