This thesis investigates how people make meaning in and from museums, through encounters with artefacts which are mediated by portable digital technologies. It provides evidence that technology can help to manage the amount of information visitors encounter, instead of increasing it, through activities which structure the use of technology. One such activity - visitor-constructed trails through museums - is studied in depth, with attention to how (and to what extent) the activity is structured, the contexts in which it takes place, and how various tools and resources mediate and support the activity. Three studies engage different types of visitors in trail construction, using mobile phones and portable digital audio player/recorders - technologies already commonly carried by visitors - in museums of art, science and history. Trails are shown to support meaning making by providing a curatorial scaffolding for visitors to recontextualise artefacts, through interpretations which are links between visitors' and artefacts' contexts, and are generally narrative in form. Technology is shown to help visitors make connections with artefacts through a two-way contextualisation, and by working in concert with other tools and resources. Meaning making is analysed using a conceptual model for the design and analysis of trails, which is grounded in a constructionist epistemology, a theoretical perspective on museum meaning making, and a methodology derived from activity theory.