This document describes the design of the PLUS (Public software Library for Ultrasound) open-source toolkit. The toolkit provides a basic infrastructure for implementing ultrasound-guided intervention systems. Functionalities include collection of synchronized ultrasound and position data from a wide variety of hardware devices, spatial and temporal calibration, volume reconstruction, live streaming to end-user applications, and recording to and replay from file. Source code, documentation, tutorials, application examples are available with a BSD-type license at the project website: www.assembla.com/spaces/plus.