The landscape of television is changing. Modern Internet enabled sets are now capable computing devices offering new forms of connectivity and interaction to viewers. One development enabled by this transition is the distribution of auxiliary content to a portable computing device, such as a mobile phone or tablet, working in concert with the television. These configurations are enabled by second screen applications that provide relevant content in synchronisation with the programme on a nearby television set. This thesis extends the notion of second screen to arrangements that incorporate multiple mobile devices working with the television, utilised by collocated groups of participants. Herein these arrangements are referred to as ‘many-screen’ television. Two many-screen applications were developed for the augmentation of sports programming in preparation of this thesis; the Olympic Companion and MarathOn Multiscreen Applications. Both of these applications were informed by background literature on second screen television and wider issues in HCI multiscreen research. In addition, the design of both applications was inspired by the needs of traditional and online broadcasters, through an internship with BBC Research and Development and involvement in a YouTube sponsored project. Both the applications were evaluated by collocated groups of users in formative user studies. These studies centred on how users share and organise what to watch, incorporate activity within the traditionally passive television viewing experience and the integration of user-generated video content in a many-screen system. The primary contribution of this thesis is a series of industry validated guidelines for the design of many-screen applications. The guidelines highlight issues around user awareness devices, content and other user’s actions, the balance between communal and private viewing and the appropriation of user-generated content in many-screen watching.