Visual markers are graphic symbols designed to be easily recognised by machines. They are traditionally used to track goods, but there is increasing interest in their application to mobile human-computer interaction (HCI). By scanning a visual marker through a camera phone, users can retrieve localised information and access mobile services. In particular the dissertation examines the application of visual markers to physical tagging: practices of association of digital information with physical items. One missed opportunity in current visual marker systems is that the markers themselves cannot be visually designed; they are not expressive to humans, and thus fail to convey information before being scanned. To address this limitation, this dissertation introduces the idea of designable markers, visual markers that are both machine-readable and visually communicative to humans, and presents an investigation of the ways in which they can support mobile human-computer interaction. The application of designable visual markers to the creation of mobile interfaces is explored through a variety of methods: through formal usability experiments, through the creation and analysis of example designs, as well as through the qualitative analysis of two field trials. All three approaches were enabled by the engineering and development of d-touch, an actual recognition system that supports designable visual markers and by its integration in a variety of applications and experimental probes. D-touch is based on image topology, and its markers are defined in terms of constraints on the nesting of dark and light regions. The constraints imposed by d-touch are flexible enough to allow novice users to create markers which are visually expressive and at the same time machine readable. A user study demonstrates how such system enables people to design their own functional visual markers, determining their aesthetic qualities and what they visually communicate to others. A desktop application to support users in the creation of valid markers, the d-touch analyser, is presented and its usefulness is demonstrated through the same study. A formal usability experiment comparing five variations of marker-based interfaces on keypad and touch-screen phones shows that all of them allow users to reliably select targets within, on average, less than 4 seconds. Participants of the experiment reported a strong preference for interfaces that involve only marker scanning, compared to those that require a combination of marker scanning and key-presses or touch selections. Example designs of mobile interface generated by the author as well as others are presented to expose how the d-touch recognition system can be integrated in mobile applications. The examples illustrate a variety of ways in which markers can be used to augment printed materials such as cards, books and product packages, adding to them interactive capabilities. The examples show also different approaches to marker design, ranging from simple and recognisable iconic design, to symbols that integrate cues about the interactive functionality, to making them invisible by hiding them in existing graphics. Finally, the dissertation reports and analyses two field trials conducted to study what practices of physical tagging can emerge from, and be supported by, the use of markers. The trials were centred around the use of uWiki, a functional prototype based on d-touch, that allows users to associate digital content to markers printed on physical tags that can be affixed to objects or buildings. Observations show that a variety of practices emerge around the use of this technology, indicating that they provide a rich medium that has potential to attract the interest of real users. Though the results of this work are preliminary, they serve to demonstrate the range of potential for the future of such systems.