We discuss liberalising the temporary mobility of workers under Mode 4 of the GATS, particularly the movement of medium and low skilled service providers between developing and developed countries. Such mobility potentially offers huge returns: a flow equivalent to three per cent of developed countries' skilled and unskilled work forces would generate an estimated increase in world welfare of over US $150 billion, shared fairly equally between developing and developed countries. The larger part of this emanates from the less-skilled, essentially because losing higher-skilled workers cuts output in developing countries severely. The mass migration of less skilled workers raises fears in developed countries for cultural identity, problems of assimilation and the drain on the public purse. These fears are hardly relevant to temporary movement, however. The biggest economic concern from temporary mobility is its competitive challenge to local less skilled workers. But as populations age and the average levels of training and education rise, developed countries will face an increasing scarcity of less skilled labour. Temporary mobility thus actually offers a strong communality of interest between developing and developed countries. The remainder of the paper looks at the GATS provisions on Mode 4 and the commitments that have been made under it. The paper reviews several official proposals for the Doha talks, including the very detailed one from India, and considers several countries' existing schemes for the temporary movement of foreign workers. Many countries have long had bilateral foreign worker programs, and some regional agreements provide for liberal and flexible movement. These show what is feasible and how concerns can be overcome. We caution that, to be useful, any WTO agreement must increase mobility, not just bureaucratise it. The paper concludes with some modest and practical proposals. We suggest, inter alia, that licensing firms to arrange the movement of labour is the most promising short-term approach to increasing temporary mobility.