There is clear consensus on the value of diversity as applied to energy systems, with the concept being a central reference for governments, industry and civil society organizations. Given its importance in policy debates, we have sought to explore the specification and measurement of diversity. We show that although conceptualisation of diversity has developed over recent years, along with increasingly elegant mathematical representations, the concept is, at core, subjective and irreducibly context specific. Subjectivity derives from determination of boundaries and the placement of objects into categories, the acts that make assessment of diversity possible. We illustrate this point with an empirical analysis of the diversity of renewable energy generation in the UK over the past century. By applying a range of different indices and classifications to this dataset, we demonstrate that the 'diversity story' told is different in each case. As such we argue that the analysis of diversity must be produced and consumed critically. Attempts to expand, manage, measure or comment upon the diversity of a system, be it an ecosystem, an organization, an economy, or an energy portfolio, demand rigor, reflexivity and, most importantly, transparency. (C) 2013 Elsevier Ltd. All rights reserved.