The disease model states that problems are symptoms of a small set of underlying disorders. This explains observable clinical symptoms by a small set of latent variables (e.g. depression). A network is a set of elements (nodes) connected through a set of relations. In network models, disorders are conceptualized as systems of causally connected symptoms rather than effects of a latent disorder. Mental disorders cannot be identified independently of their symptoms. In medicine, the medical condition can be separated from the symptoms. In psychology, this is not possible. In order to separate this, it must be possible that a person has symptoms without the disorder (e.g. depression without feeling down is not possible). In mental disorders, it is likely that there is symptom-symptom causation. One symptom causes another symptom and this leads to a mental disorder. With network systems, it might be unclear where one disorder starts and another stops. The boundaries between disorders become unclear. Network models might change treatment, as the treatment is then no longer aimed at the disorder but rather at the symptoms and the causal relationship between the symptoms.Networks in psychopathology can be created by using data on symptom endorsement frequencies (e.g. looking at correlations between symptoms) (1), assess the relationship between symptoms rated by clinicians and patients (2) and use the information in the diagnostic systems (3). In networks, any node can reach another node in only a few steps. This is called the small world property. The DSM attempts to be neutral, theoretically, but makes claims about causal relationships between the disorders. Asking experts on how nodes are related (e.g. clinicians and symptoms of a disorder) is called perceived causal relations scaling Extended psychopathology systems refers to network systems in which the network is not isolated in a single individual but spans across multiple individuals....