The dynamic properties of unentangled adsorbed polymer layers are considered theoretically. The kinetics of penetration of a chain in an adsorbed layer is considered layer is considered as a two-step process: entry corresponding to the attachment of the chain on the adsorbing surface and spreading on the surface. We discuss three types of experiments, the formation of an adsorbed layer, the desorption from an adsorbed layer and the exchange between an adsorbed layer and a bulk solution. The bottleneck for the adsorption of a new chain onto a solid wall (already covered by polymers) is the spreading stage corresponding to creation of G contacts with the wall with a total binding energy of order of kT. The desorption of a preadsorbed layer in contact with pure solvent is extremely slow and only a very small fraction of the adsorbed chains is expected to desorb within the experimental time scale. The exchange between labelled adsorbed chains and unlabelled chains in solution is slower than the formation of a saturated layer, but much faster than the desorption process. The molecular weight dependencies of the characteristic times of the three processes are predicted using the Rouse-Zimm dynamics for the polymer chains.