In many countries, the targeting of income transfer schemes leads to a very high effective marginal tax rate on private income. How can the equity goals associated with targeting be made consistent with the maintenance of labour supply incentives? This paper reviews the inevitable trade-offs facing income-tested tax-transfer systems, and then goes on to examine the conclusions of a growing body of economic analysis of these questions. This analysis, growing out of the literature on ‘optimal income taxation' seeks to provide a framework for a balancing of the conflicting efficiency and equity issues involved in income-based redistribution. Though existing research is not able to provide firm guidelines to policy, there are valuable insights - particularly from research that has begun to incorporate the administrative features of programs. These have major implications for the structure of income testing. Insofar as activity testing increases labour supply, one might argue for the use of a higher benefit withdrawal rate - since this permits a lower tax rate at other points in the distribution without defeating equity objectives. At the same time, economic theory has yet to seriously analyse the diversity of social goals in this area. Different social evaluations of the value of ‘leisure’ may have important implications for policy.