This thesis aims to provide a comprehensive theory of age discrimination that can guide the direct and indirect age discrimination provisions of the Equality Act 2010. The Act holds that unequal treatment on the grounds of age and measures that are on their face age-neutral but have the effect of disadvantaging particular age groups are lawful only if the treatment can be shown either to be a 'proportionate means of achieving a legitimate aim' or if the treatment fits into a specifically prescribed exception. In this way, the proportionality test distinguishes justified and unjustified age-differential treatment with only the former legally permissible. I outline and defend a pluralist theory of age discrimination that assists in making the distinction between justified and unjustified age-differential treatment. The theory identifies the principles that explain when and why age-differential treatment wrongs people and the principles that can justify this treatment. It is a pluralist theory because it recognises that age-differential treatment can wrong people for a number of different, overlapping reasons, and these different reasons should inform how we apply age discrimination law. The pluralist theory of age discrimination theory can improve legal reasoning in age discrimination cases by articulating the relevant principles and competing interests that are at stake in age discrimination claims. In constructing the theory, I adopt the reflective equilibrium method. This requires that I 'test' my starting moral intuitions against other beliefs, seeking coherence among these beliefs, and revising the beliefs as a result of particular challenges to them. In applying this method, I identify the following five principles to form a pluralist theory of age discrimination: equality of opportunity, social equality, respect, autonomy and efficiency.