The nearest Hilbert-Schmidt distance of (pure) states to coherent states is
discussed as a quantitative measure of nonclassicality of states in quantu
m optics or quantum mechanics of harmonic oscillators. Fock states, squeeze
d coherent states and finite superpositions of coherent states (Schrodinger
cat states) are considered as examples. Some other approaches to nonclassi
cality of states in quantum optics are shortly discussed.