Synthetic biology is a field in-the-making: a loosely-defined amalgamation of diverse disciplines, institutions, and practices. Where some practitioners identify as scientists, others consider themselves engineers; while some extol the simplicity of standardised biology, others dismiss it as counterproductive. Three different communities in synthetic biology (epistemics, sceptical constructors and committed engineers) can be distinguished by way of their intentions, practices and promises.Synthetic biologists' promises shape policy-makers' expectations, which in turn shape institutional arrangements. These institutional arrangements then influence practitioners' promises in an iterative fashion. In both the US and the UK, 'committed engineers' have succeeded in gaining support for an engineering-based and industry-centred vision of synthetic biology, which promises applications and economic growth. This group's intentions and promises have influenced policy-makers' expectations, which, in turn, have driven the major institutional developments in synthetic biology in the two countries.However, while the promises of the economic potential of this vision of the field have been embraced at policy levels, other aspects of this vision, such as the importance of enabling infrastructure, are often overlooked. In a sense, committed engineers' promises and rhetoric have been too successful, because they have overshadowed the institutional and infrastructural developments needed to make them a reality.