Wearable sensors provide a means of continuously monitoring a person in a natural setting. These sensors can “look in” by monitoring the wearer’s health through physiological measurements and also by detecting their activities. Other sensors can be used to “look out” from the wearer into the environment through which he/she is moving, which may serve to detect any potential hazards or provide contextual information about the wearer’s lifestyle. Wearable sensors can be harnessed to give immediate feedback to the wearer while also providing an archive of physiological data which can be logged and assessed over days, months or years. This has many applications in the field of healthcare, rehabilitation and sports performance. Here we present a number of case studies involving “smart” garments which have been developed to monitor the well-being of the wearer and to assess performance and progress, for example in training or rehabilitation scenarios.