During the early user interface design phase, different solutions should be explored and iteratively refined by the design team. In this rapidly evolving scenario, a tool that enables and facilitates changes is of great value. UISKEI takes the power of sketching, allowing the designer to convey his or her idea in a rough and more natural form of expression, and adds the power of computing, which makes manipulation and editing easier. More than an interface prototype drawing tool, UISKEI also features the definition of the prototype behavior, going beyond navigation between user interface containers (e.g. windows, web pages, screen shots) and allowing to define changes to the state of user interface elements and widgets (enabling/disabling widgets, for example). This dissertation presents the main concepts underlying UISKEI and a study on how it compares to similar tools. The user interface drawing stage is detailed, explaining how the conversion of sketches to widgets is made by combining a sketch recognizer, which uses the Levenshtein distance as a similarity measure, and the interpretation of recognized sketches based on an evolution tree. Furthermore, it discusses the different solutions explored to address the issue of drawing an interaction, suggesting an innovative mind-map-like visualization approach that enables the user to express the event, conditions and actions of each interaction case while still keeping the pen-based interaction paradigm in mind.