This book reviews the different theories and models that seek to explain the causes of depression from different perspectives, from the molecular to the socio-cultural level. Depression is a complex psychopathological construct of high phenotypic heterogeneity, which must be understood as a phenomenon in which different explanatory levels interact with each other. However, very little is known about this interaction. The aim of this book is to provide clinical psychologists and psychiatrists a better knowledge of the interaction of different etiopathogenic levels, in order to help these professionals make better therapeutic decisions when treating depressed patients.