Negative symptoms are relatively frequent across schizophrenia spectrum disorders diagnostic categories and they represent deficits in different domains such as emotional, volitional and experiential. Even though negative symptoms have long been recognized as a core feature of schizophrenia, their definition has been changing over time. Different conceptualization classified this category of symptoms as primary or secondary, persistent or transient. At the current moment there are five agreed upon domains of the concept of negative symptoms, which are separated into two dimensions—experience (anhedonia, avolition, asociality) and expression (blunted affect, alogia). Multiple mechanistic pathways have been proposed and investigated for each dimension and for each domain. The current chapter attempts to address recent advances in the literature regarding the concepts, definitions and classifications of negative symptoms and their etiological model.