In this tutorial review, different film microstructures, commonly termed morphologies, into which the organic semiconductor polymers self-assemble macroscopically are presented, together with their corresponding influence on charge carrier mobility and hence transistor behaviour. It will be clarified how various chemical design approaches and solution processing methods enable the manipulation of polymer morphology, leading to improvements in transistor performance. Ultimately, it is illustrated that the directional alignment of polymers form oriented fiber-like films, yielding one of the highest mobilities reported so far for polymer transistors. Based on these observations, a prediction is made concerning which kind of morphology is expected to reach the best charge carrier mobility.