Deep brain stimulation involves using a pacemaker-like device to deliver constant electrical stimulation to problematic areas within the brain. It has been used to treat over 40,000 people with Parkinson’s disease and essential tremor worldwide and is currently undergoing clinical trials as a treatment for depression and obsessive–compulsive disorder. This article will provide an historical account of deep brain stimulation in order to illustrate the plurality of interests involved in the development and stabilization of deep brain stimulation technology. Using Latour’s notion of immutable mobiles, this article will illustrate the importance of clinical assessment tools in shaping technological development in the era of medical device regulation. Given that such tools can serve commercial and professional interests, this article suggests that it is necessary to scrutinise their application in research contexts to ensure that they capture clinical changes that are meaningful for patients and their families. This is particularly important in relation to potentially ethically problematic therapies such as deep brain stimulation for psychiatric disorders.