Much of the developed world is currently affected by an overweight epidemic. A large population of people are undertaking a personal fitness regime in order to improve fitness, lose or maintain weight or for general health reasons. Conventionally, the best method to achieve the greatest results was often to obtain the services of a personal trainer. These fitness professionals are educated in physiology, exercise science and motivational methods, and use a combination of these expertise to design and assist in the performance of a tailored exercise program. With the advent of Smart Phones and affordable sensing technologies, the possibility to automate several of the functions of a personal trainer has emerged. These "Virtual Trainers" allow a person to manage their own personal fitness program while integrating close monitoring, sports science knowledge and motivational aspects - several of the same services a personal trainer will provide. This paper discusses several of the most widely used "Virtual Trainer" systems available today. (C) 2012 Published by Elsevier Ltd.