Abstract This chapter starts with a brief history of the development of external “tabletop” pacemakers from the late 1920s through to the introduction of the transistor in the early 1950s when Bakken and Hermundslie (Medtronic) developed the first battery operated portable device in the United States. This was followed shortly afterwards by the development and implantation of the first internal pacemaker by Senning and Elmqvist in Sweden. The second part of the chapter documents improvements to the basic device including the introduction of flexible electrodes, improved batteries and electronics (leading to miniaturisation) as well as sensing and signal processing, allowing for demand and activity moderated pacing. The final section considers the modern pacemaker in terms of its individual components: pulse generation, sensing, output electronics, timing, telemetry, power sources, leads, and programmers.