The motivation and goals for the development of mobile health solutions are presented. Short history and background of the electrocardiography provide basic principles and facts related to the heart activity and elucidate the path from the measurement of conventional electrocardiogram to the monitoring of the heart by wireless body sensors. A bipolar differential lead is introduced, which has enabled the implementation of a small body sensor for capturing electric potential differences on the skin. The chapter concludes with guidelines on how to read this book and with a short overview of all book chapters.