There is an increasing number of buildings that require informed decisions to be made about their continued safety and serviceability. Although social and economic issues are often all-important influences, the technical issues nevertheless need to be addressed objectively, efficiently and reliably. This book shows how monitoring the physical behaviour of a structure can assist the engineer to meet these conditions when making an assessment. The book is aimed primarily at the practising engineer charged with making recommendations in respect of safety and serviceability. By the same token, it will be of value to the client specifying a brief for assessment or evaluating the report of an investigation which involves monitoring. The book will also be one of reference for those engaged in research involving monitoring, and an aid to the advanced student who needs to understand better the full-scale performance in service of building structures. The need to assess safety and serviceability may arise for a variety of reasons, ranging from problems developing in service to change of use or the introduction of innovative features at the design stage. These reasons are explored in the first chapter which establishes a philosophy by which the assessing engineer can determine appropriate courses of action. Observations and measurements which do not address the real issues are worthless but too much information which cannot be effectively digested and interpreted is also not useful.