The introduction illustrates the problems of thin film flow and their stability or instability with two examples. The first is the classic Landau-Levich-Derjaguin problem of the coating of a plate withdrawn from a bath of liquid. This example allows to explain how well experiment and theory can be linked together and help in understanding this problem. The second concerns a much more complex situation: dewetting of a film made up from two liquids in a finite geometry. In this case, «only» experiments are described since the system shows already a high level of complexity.