In modern clinical diagnostics, the electrocardiogram (ECG) is an important medical tool. The so-called 12-lead ECG is widely used today, however such systems can be very expensive, large and heavy. This work describes the development of a low-cost 12-lead ECG system, illustrating the design process of the hardware and software architecture, as well as techniques for the reduction of noise and interference in ECG signals. In this thesis, ECG signal measurements are analyzed in the time domain as well as the frequency domain. Digital signal processing techniques such as adaptive filtering or sample-rate conversion are utilized. For the hardware design of this 12-lead ECG system, the heart rate monitor front-end integrated circuit AD8232 is used. The results show that this form of ECG system can compete with a high-end ECG research amplifier. The power-line interference can be significantly reduced with the help of an adaptive filter, and the baseline drift can be reduced by the use of a sample-rate-conversion process. This work is a foundation for future research in building ECG systems, and noise reduction in ECG signals. Such ECG systems could be used as wearable devices, for various applications such as 24-hour ECG, or medical research.