ABSTRACT
Macroscopic systems being composed of atoms and fields, there is no reason why they should not obey the quantum laws. It therefore seems quite natural to consider them as quantum systems, albeit extremely complex ones. Measurement constitutes a riddle, and a great many theories were put forward as attempts to solve this riddle. The number is so considerable indeed that to try and review them all would be an almost impossible undertaking. A few essentially quantum notions, such as quantum tunneling, have been shown quite indispensable for explaining a number of experimental facts concerning some systems of macroscopic size and complexity. Small differences necessarily exist between a system as it really is and its description by means of simple statements Moreover, elaborate experimental setups can in general be conceived of that, if used as measurement devices, would enhance these small differences.
