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These notes contain an extended version of lectures given at the ``SummerSchool on Large Coulomb Systems'' in Nordfjordeid, Norway, in august 2003. Theyfurnish a short introduction to the theory of quantum harmonic systems, or freebose fields. The main issue addressed is the one of local states. I will adoptthe definition of Knight of ``strictly local excitation of the vacuum'' andwill then state and prove a generalization of Knight's Theorem which assertsthat finite particle states cannot be perfectly localized. It will furthermorebe explained how Knight's a priori counterintuitive result can be readilyunderstood if one remembers the analogy between finite and infinite dimensionalharmonic systems alluded to above. I will also discuss the link between theabove result and the so-called Newton-Wigner position operator therebyilluminating, I believe, the difficulties associated with the latter. I will inparticular argue that those difficulties do not find their origin in specialrelativity or in any form of causality violation, as is usually claimed

Local States of Free Bose Fields