Poor people participate in markets as both consumers and providers. Markets work for the poor when poor people as consumers become able to access a desired product or service provided through a market. But, in any particular good or service market, what does effective access mean and who has it? How much will levels of access change in response to policy action or providers' strategies? Market-based solutions may well not reach all eligible people, in which case, what are the limits of the market and who is excluded? To answer questions like these, policy makers and providers need to be able to define and measure access both now and in the future. The access frontier approach described in this article provides a practical way of doing this. The approach rests on the segmentation of the entire potential market for a good or service into different zones using both demandand supply-side information. This article demonstrates the applications of the access frontier in a financial service market - transactional banking in South Africa - and demonstrates its implications for policy development and market strategy.