Abstract Polymer electrolyte membrane (PEM) fuel cells are a promising technology for automotive applications. To achieve the cost versus durability required for the commercialization of this technology, material production and its associated challenges have gained much interest. To reduce the production time per fuel cell stack, it is important to increase the number of units produced. A time-consuming step in the production of stacks is the activation, also known as break-in, which is necessary to carry out a subsequent factory-acceptance-test. The state-of-the-art studies found in literature are mainly tailored towards investigating various break-in procedures without taking into consideration the possible mechanisms behind the performance increase during the initial operation. This interconnection between break-in procedures and physical phenomena is hence missing. In this review, we describe the optimized state for the membrane and catalyst layer in regards to their morphology and composition. We compare this to the known state after production and discuss which mechanisms change the initial state. This information is then used to put into perspective the mechanisms that improve the cell performance and the time scale on which they will take place. Despite the high dependency of the activation behavior on the production steps and the material used, we can conclude that the main sluggish activation mechanisms for state-of-the-art CCMs are the removal of solvents from production and changes in the catalyst layer ionomer and at the membrane surface. Membrane bulk protonic conductivity and changes in platinum structure are expected to have a subordinate role in the activation process. High humidities or even liquid water in the cell and the cycling between oxidizing and reducing conditions at the electrodes accelerate the activation process. Thus, this review serves the development of “smart” and “fast” break-in procedures.