Testing is a key part of any systems engineering project. There is an extensive literature on testing, but very little that focuses on how testing is carried out in real-world circumstances. This is partly because current practices are often seen as unsophisticated and ineffective. We believe that by investigating and characterising the real-world work of testing we can help question why such ‘bad practices’ occur and how improvements might be made. We also argue that the testing literature is too focused on technological issues when many of the problems, and indeed strengths, have as much do with work and organisation. In this paper we use empirical examples from four systems engineering projects to demonstrate how and in what ways testing is a cooperative activity. In particular we demonstrate the ways in which testing is situated within organisational work and satisfices organisational and marketplace demands.