OBJECTIVE To explore how and why self-monitoring of blood glucose (SMBG) is carried out in a real-world context. METHODS We conducted a multicase study among ten people with type 1 and insulin-treated type 2 diabetes mellitus in Scotland, alongside seven nominated support people and four healthcare professionals. All participants were interviewed in depth and six participants provided SMBG diaries. Stones' version of structuration theory informed the analysis. RESULTS People with diabetes were able to provide immediate motives for SMBG at particular times, often having different motives on different occasions. We identified six such motives, including routine, in response to symptoms, associated with a diabetes review, to facilitate lifestyle, when a 'good' result was expected, and higher level motives for longer term glycemic control. These motives were influenced by underlying attitudes toward diabetes that included level of engagement and responsibility for diabetes, a desire not to be controlled by diabetes, resistance to diabetes, diabetes education and relationship with the health service, fear of hypoglycemia, and prevention of diabetes complications. Five responses to test results were identified, depending on the immediate motive and underlying attitudes. CONCLUSIONS People with insulin-treated diabetes do not necessarily self-monitor with an explicit goal of improving long-term glycemic control, but may have other motives that are important to them. An individualized understanding is therefore needed to advise people with diabetes how SMBG can be optimized for them.