  The link between the nucleon generalized parton distributions and the
non-diagonal one-body density matrix in momentum space is studied. Attention is
focussed on the region where quark generalized parton distributions (GPD's)
describe emission and reabsorption of a single active quark by the target
nucleon. The correct covariant connection with wave functions used in any
constituent quark model is established. Results obtained with different
constituent quark models are presented for the unpolarized quark GPD's.
