Abstract
This book concerns original and innovative developments on the notions of observability and opacity of dynamic systems evolving in space and time (so-called distributed systems). In general, and for any given system, the first thing a decision-maker does is to observe, to take measurements, so as to be able to choose the actions to be applied to achieve a given objective. How to take measurements, the relevance and performance of these measurements, and what they do and don't allow us to observe, is the subject of this book.The ideas explored form the basis of all developments and actions for the management of a real system. This applies equally to ecological and environmental systems, health and human systems, and engineering in general.