The first in a series of briefing papers on inequality, this paper looks at what is meant by 'inequality'. It argues that there are several dimensions to inequality and that general analyses often miss those that are most affected by it. The paper suggests that the empirical data on inequality is comparatively weak and that research in the area would benefit from longitudinal studies to distinguish between transient and chronic components of inequality. The paper then moves on to review various methods for measuring inequality and concludes that while conventional techniques are useful and valuable, it is important to broaden our understanding of inequality by incorporating qualitative indicators and developing a multidimensional approach. Finally it suggests that getting to grips with the factors and processes behind inequality is key to formulating policies to mitigate and reduce it.