For more than ten years Pulsed Laser Deposition (PLD) has been used to produce Y1Ba2CU3O7-8 (YBCO) thin films. Most groups have reported their capacity to produce films with roughly optimum characteristics. Unfortunately, the deposition parameters used by the individual groups vruy within broad limits [1-5]. Each group develops a similar but unique recipe of deposition parameters, which, combined with other system specific characteristics, optimises their film properties. The implication of this is that the set of parameters being reported is not the whole story. In this paper we put forward a set of generally accepted facts about the PLD process and we list the necessary implications of these facts. To account for the facts we suggest a model based upon the evolution of a collisioinal layer at the surface of the film and the selective retention of impringing species.