The pig sector undergoes a major transformation characterized by intensification, geographic concentration, vertical integration and increasing scales of production. This creates new challenges for animal welfare. Sensor technology offers multiple advantages for monitoring key indicators of welfare in pigs that can help addressing these new challenges in the pig sector. Technology allows continuous and objective monitoring on individual and group level, on multiple farms and also in various environments. Real-time implementation of sensor based monitoring systems makes it possible to detect a problem and to take immediate action to improve the life of the animal under consideration. This chapter reviews recent advances in technologies for monitoring key indicators of pig welfare by considering good feeding, good housing, good health and appropriate behaviour. It presents how accelerometer based nest-building detection system can contribute to improvement of pig welfare and major future trends in development of monitoring technology.