Virtual sensors are “software simulations” of real sensors based on measurements of other sensors and a model. This paper presents a new design software which supports the creation and parametrization of modular virtual sensors, i.e. virtual sensors consisting of several “subsystems”. The workflow, the core concepts, the plug-in interfaces for user-specific extensions, and an example are presented. The main highlights of the proposed software are the possibilities to automatically invert (sub)systems—allowing efficient parameter estimation—, and to automatically generate several proposals for virtual sensors out of a larger physical model.