Wearable and smartphone-based activity and heart rate (HR) monitors are becoming increasingly common, with around 80 million devices in use in 2017. Wearable and smartphone-based devices may be dedicated HR monitors or part of an activity tracker system. One of the main aims of these devices is to encourage exercise and increase fitness, which is clearly desirable in a society with high levels of inactivity and obesity. These devices provide individuals with large amounts of data including HR information. This may, therefore, give an opportunity to document or diagnose arrhythmias. Undiagnosed atrial fibrillation is a common problem and is associated with a huge burden of potentially preventable stroke. Wearable HR monitors may provide the opportunity to identify these individuals and allow them to receive stroke prevention treatment. However, the consumer fitness market is unregulated and the manufacturers emphasise that their devices are not intended to be used for detecting heart rhythm problems. The reliability, sampling frequency and algorithms for the HR data these devices provide are hugely variable. Detected ‘abnormalities’ may inform clinical decision making but it may also trigger unnecessary anxiety and costly investigations in healthy people.