A recurrent theme in the characterization of synthetic biology is the role of engineering. This theme is widespread in the accounts of scholars studying this field and the biologists working in it, in those of the biologists themselves, as well as in policy documents. The aim of this article is to open this black-box of engineering that is supposed to influence and change contemporary life sciences. Too often, both synthetic biologists and their critics assume a very narrow understanding of what engineering is about, resulting in an unfruitful debate about whether synthetic biology possesses genuine engineering methodologies or not. By looking in more detail to the diversity of engineering conceptions in debates concerning synthetic biology, a richer perspective can be developed. In this article, I will examine five influential ways in which engineering is understood in these debates, namely engineering as applied science, as rational methodology, context-sensitive practice, cunning activity or design. The claim is first of all thus to argue that engineering must not be seen as something stable or characterized by a fixed essence. It rather has multiple meanings and interpretations. Secondly, the claim is that most of the debates on synthetic biology cannot be indifferent towards the question which conception of engineering is at play, since the specific questions and concerns that pop up depend to a great extent on the precise conception of engineering one has in account. Many of the existing debates around synthetic biology can thus be reinterpreted and readdressed once one is aware of which conception of engineering is at play.