The advancement of mobile devices, battery longevity, and low cost sensors is a confluence of enabling technologies, which have led to the creation of self-monitoring systems. This paper is a review of technologies within the field of self-monitoring or Quantified Self systems. Additionally, an original framework is presented for classifying such systems. The framework is intended to aid in evaluation of existing or proposed systems, managing research, and applications of self-monitoring systems. Technology gaps and applications are then addressed to highlight what topics are under-developed or covered by existing work. Knowing these topics could then be used to direct research and development to a certain degree.