In using the term 'hybrid interactions', we refer to interaction forms that comprise both tangible and intangible interactions as well as a close coupling of the physical or embodied representation with digital output. Until now, there has been no description of a formal design process for this emerging research domain, no description that can be followed during the creation of these types of interactions. As a result, designers face limitations in prototyping these systems.

In this thesis, we share our systematic approach to envisioning, prototyping, and iteratively developing these interaction forms by following an extended interaction design process. We share our experiences with process extensions in the form of toolkits, which we built for this research and utilized to aid designers in the development of hybrid interactive systems.

The proposed tools incorporate different characteristics and are intended to be used at different points in the design process. In Sketching with Objects, we describe a low-fdelity toolkit that is intended to be used in the very early phases of the process, such as ideation and user research. By introducing Paperbox, we present an implementation to be used in the mid-process phases for fnding the appropriate mapping between physical representation and digital content during the creation of tangible user interfaces (TUI) atop interactive surfaces. In a follow-up project, we extended this toolkit to also be used in conjunction with capacitive sensing devices. To do this, we implemented Sketch-a-TUI. This approach allows designers to create TUIs on capacitive sensing devices rapidly and at low cost. To lower the barriers for designers using the toolkit, we created the Sketch-a-TUIApp, an application that allows even novice users (users without previous coding experience) to create early instantiations of TUIs.

In order to prototype intangible interactions, we used open soft- and hardware components and proposed an approach of investigating interactivity in correlation with intangible interaction forms on a higher fdelity. With our fnal design process extension, Lightbox, we assisted a design team in systematically developing a remote interaction system connected to a media façade covering a building.

All of the above-mentioned toolkits were explored both in real-life contexts and in projects with industrial partners. The evaluation was therefore mainly performed in the wild, which led to the adaptation of metrics suitable to the individual cases and contexts.