Gamma rays from molecular clouds

High energy gamma-rays from individual giant molecular clouds contain unique information about the hidden sites of acceleration of galactic cosmic rays, and provide a feasible method for study of propagation of cosmic rays in the galactic disk on scales less than or equal to100 pc. I discuss the spectral features of pi(0)-decay gamma-radiation from clouds/targets located in proximity of relatively young proton accelerators, and speculate that such `accelerator+target' systems in our Galaxy can be responsible for a subset of unidentified EGRET sources. Also, I argue that the recent observations of high energy gamma-rays from the Orion complex contain evidence that the level of the `sea' of galactic cosmic rays may differ significantly from the flux and the spectrum of local (directly detected) particles.