The observed widths of molecular lines indicate that molecular clouds are in a highly turbulent state with supersonic internal velocities. At the same time molecular clouds are highly fragmented into substructures exhibiting large density contrasts. These substructures seem to be arranged in a hierarchical order scheme where one can think of newly formed stars representing the smallest spatial scale of ordering. Stars, protostars and even dense clumps can be regarded as being hydrodynamically decoupled from the ambient gas. Thus, in an idealized way, one can think of molecular clouds as being a gravitationally coupled system consisting of gas (continuous component) and ‘point-masses’ (particle component) randomly moving through the gas. The random motion of these ‘point-masses’ is accompanied by spatial and temporal fluctuations of their gravitational potential which, in turn, induce velocity and density fluctuations of the gas.