What are scenarios?It is impossible to predict, with absolute certainty, what will happen tomorrow, let alone fifty years hence. Our lives are subject to myriad influences, physical and social forces that shape both the world we live in and the relationships between actors. In a city the size of London, the number of individuals and organisations is enormous, and the number of interactions is probably beyond calculation.Thinking about the future in terms of current trends is a common approach. For instance, the introduction to this book includes predictions about global population, forecasts about how many of those people will live in London and projections about the implications for the capital's housing, transport and education sectors. These figures are important and thought provoking, they frame debate and help identify priorities. However, simply following trends does not allow for the possibility of substantive, disruptive, change (Lombardi et al, 2012).When setting out policies that will determine the development of London's housing, flood defences, transport infrastructure or energy supply, it is not enough to say that the future is complex and unknowable. How can we deal with decisions made in an uncertain present that will have long-term consequences, extending into an even more uncertain future?Even if we can't make accurate predictions or forecasts about the future, we can at least be clear about our assumptions, our decisions and their likely consequences. One way to do this is to consider different 'scenarios' . Scenario analysis has been defined as 'a tool for ordering one's perceptions about alternative future environments in which one's decisions might be played out. Alternatively: 'a set of organized ways for us to dream effectively about our own future' (Schwartz, 1991).Scenarios are stories, descriptions of alternative destinations, a medium through which to share ideas about possible futures. The scenarios approach can embrace both scientific inquiry about