Abstract. Multi-fidelity prototyping combines within a single user interface prototype several elements whose representations are reproduced with different levels of fidelity with respect to the final user interface: no fidelity at all, low fidelity, medium fidelity, and high fidelity. In order to effectively and efficiently support multi-fidelity, an original set of techniques is defined and discussed: multiple representation manipulation by sketching and gesture recognition, smooth transition between any representation at any time, prototype reusability, multi-platform support, and multiple user interface prototyping facilities. The full implementation of these techniques in prototyping software provides designers and developers with a unique environment for exploring multiple designs with unprecedented support for quickly designing interfaces from scratch or from previously existing design templates. An experimental study reveals that the multiple representation manipulation together with smooth transition represents a valuable advantage for naturally designing user interfaces. The prototyping software supports several aspects involved in the user interface development life cycle and is convenient for non-WIMP user interfaces.