“The crises we face are systemic in nature. To overcome those crises we need to understand how systems work. To arrive at such an understanding we need to think systemically.” 
Throughout the twentieth century, much of psychiatry aspired to reductionist simplicity. Whether in the Freudian unconscious, the human genome, or dysfunctional neurobiology, researchers sought to identify the underlying cause of the troubles faced by their patients. However, far from uncovering simple etiologies, the past century of psychiatric research has revealed systematic complexity, leading to a growing recognition that mental disorders are dauntingly complex phenomena [2, 3]. Yet, despite this growing acceptance, there has been little change in how we study psychopathology, and much of our research remains stubbornly rooted in the monocausal framework. If we are to make genuine progress in explaining, predicting, and treating mental illness, it is critical that we embrace the complexity inherent in these disorders in our theories, methods, and empirical research.
In this collection, we present a series of papers that take up this charge. From these contributions, a central theme emerges: mental disorders arise from a host of components across biological, psychological, and social levels of analysis, intertwined in a web of complex interactions. To understand mental disorders, it is not sufficient to understand single components alone: we cannot hope to understand depression from studying only the amygdala any more than we can understand the weather by careful examination of water droplets. We must go further and understand both the individual components and the complex interactions among them. That is, we should aim to understand the systems from which psychopathology emerges.
Achieving such understanding will be no easy task. The systems giving rise to mental illness are massively multifactorial, and there is little evidence that any one factor is either necessary or sufficient for any given disorder to emerge . The systems exhibit both multifinality (i.e., the same set of causal agents can lead to different mental health outcomes) and equifinality (i.e., diverse causes can produce the same disorder) [4, 5]. The systems are heterogeneous, with individuals differing not only in the specific constellation of symptoms experienced, but also in the etiological factors that precede and predict illness onset [6, 7]. Further, systems within individuals are dynamic: they rise and fall and evolve over time.
In the six articles inaugurating this collection, researchers operating from a range of perspectives review what we know about the systems that affect mental health. In doing so, they chart a path forward for how we can approach the challenging task of better understanding these complex systems, and how we can leverage that understanding to improve treatment.