Continuous surveillance of critical parts and locations in buildings using various sensor techniques should be an essential part of future building diagnostic and maintenance systems. Up until now, such ambitions have been confined to relatively few buildings world wide, mainly inhibited by the difficulties and costs of the installation, particularly in existing buildings. Recent advances in the field of information technology (IT) have highlighted the possibility of making building monitoring and control systems conceivable to a larger group of users. Novel sensors and measurement techniques enable monitoring of almost any part of a building in terms of moisture content, relative humidity, temperature, ventilation, etc. Local networks, communicating over any physical media, e.g. twisted pair, power lines or even wireless, connect the various measurement nodes together. On-line processing and evaluation of relevant data can provide the user with almost instant information on the status of buildings via the Internet or other modes of telecommunication. The paper reviews and exemplifies the experience of the Centre in the area of building monitoring in general and of historic buildings in particular. Ideas and speculations about future development of web-based maintenance systems for historic buildings will be discussed.