This chapter introduces the field of discrimination law, explaining why it takes the form it does and summarizing the critical arguments often advanced concerning the whole body of anti-discrimination legislation. The scope of anti-discrimination law has widened very considerably over the past twenty years, principally as a result of EU law protecting people on grounds such as age, sexual orientation, religion, and fixed-term or part-time status. Different areas of discrimination law vary in respect of possible defences when an alleged act of unlawful discrimination has taken place. How far positive discrimination is lawful also varies. Some argue that anti-discrimination law may harm those it aims to protect by distorting the market and discouraging the hiring of under-represented groups. There is much debate about whether it is possible to establish a general principle to help define who exactly should be protected by discrimination law, in what ways, and on what basis.