© 2019, Springer Nature Switzerland AG. Introduction: Harms from medical devices are important, but have been much less well studied than adverse drug reactions. Information provided to device users is of variable quality. Objective: Our aim was to define “medical device fault” and “adverse effect of a medical device”; to establish whether medical device faults arise in design, manufacture, or use; and to consider ways of mitigating the adverse effects of medical devices. Methods: We analysed 100 consecutive faults reported by the US Food and Drug Administration (FDA) and 50 faults reported by the UK Medicines and Healthcare products Regulatory Agency (MHRA), and classified faults according to the point at which they occurred. Results: Nearly 70% of reported faults related to devices that entered the body. Over 70% arose at the design stage, a quarter of the faults were associated with manufacture, and less than 5% were primarily caused by faulty use. Conclusion: We defined a medical device fault as an unintended failure in the design, manufacture, or use of a medical device that leads to, or has the potential to lead to, harm to the patient, and an adverse effect of a medical device as an unintended and appreciably harmful effect, caused by a medical device, which demonstrates a hazard of the device and may warrant preventive measures, or a change in the mode of use, or withdrawal of the device. Most faults that generate warnings arise from problems at the design stage, some arise at the manufacturing stage, and a few in usage. Careful assessment of the design of safety-critical devices in the light of previous problems may help to prevent repetition of errors. It would be helpful if, in addition to user manuals, manufacturers were required to produce Summaries of Device Characteristics (SDCs, “labels”) that contained a systematically presented set of information about a product.