The purpose of this paper is to discuss two forms in which the time–energy uncertainty relation can appear. One of them concerns the time of preparation of a given state and the other the lifetime of compound systems. Both will be illustrated here through exactly solvable examples. The first one will be the opening and subsequent closing of a shutter on which we have an incident beam of particles. This problem shows a diffraction‐in‐time effect closely connected with the first type of time–energy uncertainty relation. The second will be the decay of a compound state described through a schematicRmatrix formalism. There one can clearly see the transition between the time–energy uncertainty relation as associated with the interval in which the decay process has been in operation, and as associated with the lifetime of the compound state.