The effect of an ambient gas on the expansion dynamics of laser ablated plasmas has been studied for two systems by exploiting different diagnostic techniques. First, the dynamics of a MgB2 laser produced plasma plume in an Ar atmosphere has been investigated by space-and time-resolved optical emission spectroscopy. Second, deposition rate and fast ion probe measurements have been used to study the plume propagation dynamics during laser ablation of a silver target, over a large range of Ar background gas pressures (from high vacuum to approximate to 100 Pa). A comparative analysis of the experimental results allows us to identify different regimes of the plume expansion, going from a free plume at low pressure, through collisional and shockwave like hydrodynamic regimes at intermediate pressure, finally reaching a confined plume with subsequent thermalization of the plume particles at the largest pressure of the background gas. The experimental findings also show that a combination of complementary techniques, like optical emission spectroscopy, close to the target, and fast ion probe and deposition rate measurements at larger distances, can lead to a more detailed understanding of the laser ablated plasma plume propagation in a background gas. (C) 2003 Elsevier B.V All rights reserved.