Brain activity can be influenced by a person's mental and emotional condition. Therefore, the psychological state should be taken into account when investigating applications which are controlled by or dependent on brain-activity, such as Brain-Computer Interfaces (BCI). In their Model of BCI control, Kübler and colleagues [1], summarized the components which most likely influence and predict BCI performance and presented psychological factors as one such component. Psychological factors that may influence BCI performance are attention, concentration, motivation, or visuo-motor coordination. We present the state of the art with respect to the influence of psychological factors on BCI performance and discuss the importance of the BCI training situation. We argue for considering psychological aspects in BCI research, development, and translation to end-users.