Is horizontal equity (HE) the "most widely accepted principle of equity"? Or does it stand in "opposition to the advancement of human welfare"? This paper argues that the case for the HE principle is not as straightforward as is usually thought and that it requires advanced notions of justice and wellbeing. The most likely ethical basis for HE appears to combine a Rawlsian maximin principle and a view of well-being that allows for relative local comparison effects. The paper also explores some of the dimensions of equality and well-being along which the HE principle can be applied and presents a number of examples showing how HE considerations can provide an important input into policy analysis.