This chapter defines basic income and examines the dimensions that underpin it. It focuses on three principles that are fundamental to a basic income, and which relate to the potential impact of a basic income on job search behaviour amongst unemployed people. The three principles are unconditionality, universalism and non-withdrawal of benefits. From these core principles, three welfare mechanisms are identified, which link basic income to existing welfare regimes. It is from this link between theoretical principles and welfare mechanisms that the analysis throughout this book is developed. The importance of this chapter and of the book more broadly is in linking abstract theoretical terms, such as universalism, to empirically grounded welfare mechanisms that shape existing welfare regimes and their impacts on behaviour. By developing these links, we gain a better understanding of how basic income would function, its potential impact on job search behaviour and labour market participation more broadly.