This paper presents the design and implementation of Virtual Instrument Scripting Toolkit (VIST), a toolkit for constructing virtual instruments. Virtual instruments are handheld computer applications that help probing environmental properties and augmenting user interactions within a virtual environment. The toolkit, VIST, is intended to simplify the creation of components of virtual instruments and to make it possible to quickly prototype a virtual instrument from those components. VIST also provides a mechanism that allows detecting and displaying attributes of virtual objects and environmental properties in a simulated virtual environment. This paper describes the design rationales and architecture that VIST provides, and demonstrates how VIST has been used to build virtual instruments.