CustusX version 3 is an in-house application for research in image-guided therapy with a focus on intraoperative use and ultrasound imaging. It is built upon open source toolkits such as the Insight Toolkit (ITK), the Visualization Toolkit (VTK), Qt and others. CustusX has a layered architecture based on services, and is extensible through the use of plugins. Key features are a user interface configurable by the user, an abstraction of the VTK pipeline, cross-platform video acquisition including temporal calibration, and a patient model. This paper gives an overview of the architecture and a description of the key concepts.