The problem of the conceptual and formal characterization of observables in quantum mechanics is of central importance. The very name 'observable' rests on the view that one deals with observable, i.e. measurable quantities.This view gave way to a more subtle analysis, involving a distinction between observables and their measurement. Observables are those quantities which characterize a system intrinsically. They are not measurable in a naive sense. Often their value is accessible but only indirectly and through extended theoretical reasoning. A careful examination of classical mechanics shows that even there say the position of a particle cannot be determined. Measurements never tell if a particle's site has rational or irrational coordinates, they give merely a n interval for it. I n quantum mechanics the matter is still more intricate.Independently of the difficulties associated with measurements, one can define observables with regard to the kinematical group of a quantum system (e.g. the Galilei group), namely as 'suitably' transforming operators under an action of this group. For example, operators for position and momentum can be characterized by their transformation properties under Galilei symmetries in both classical and quantum mechanics without appealing to the correspondence principle. Outside this particular situation one gets into difficulties, since the word 'suitably' has then no specified meaning.