An estimated 2 million new teaching positions need to be created worldwide if the international effort to ensure that all children have access to free primary education by 2015 is to be a success.
The primary education target is a United Nations Millennium Development Goal. The next set of goals is currently being debated.
The Global Campaign for Education is leading a drive to hold governments to their pledges on increasing access to education. Last month, it said that increasing the number of trained teachers was of the greatest importance.
But the problem is starkly illustrated by student-teacher ratios in some of the least developed and post-conflict countries. A 2011 Unesco report - which provides the most recent comprehensive figures - found that 96 out of 195 countries needed to expand their teaching forces before all children would be able to attend primary schools.
Many countries in sub-Saharan Africa have ratios of close to 1:40. In Madagascar, Mozambique, Sierra Leone and Togo, ratios sometimes exceed 1:80.