Schizophrenia Spectrum Disorders (SSD) manifest multidimensionally, presenting a syndromic nature with several symptomatic domains. These are driven by dynamic systemic biological changes that unfold over the course of the disease. Thus far, the diagnosis is solely based on symptoms, which may be rather subjective, moving research toward the search for SSD biomarkers. This book presents a summary of the main hypotheses that have evolved over time to explain the pathophysiology of SSD and that have driven the discovery of associated biomarkers: neurotrophic, neurotransmitter, neuroendocrine, immune-inflammatory, nitrosative/oxidative stress, metabolic, and gut microbiota-brain axis. The book shows the most relevant research carried out in the last twelve years to develop predictive, diagnostic, theranostic or transdiagnostic models based on these biomarkers using the receiver operating characteristic curve (ROC) analysis. Chapters also discuss how the literature has addressed the development and/or addition of new discriminatory biomarkers to achieve robust and successful results. The book is rounded out with a step-by-step explanation on how to work on the MetaboAnalyst platform, including the meaning of the chosen statistics and how to interpret them in the results obtained. This book is a useful resource for students and scientists involved in the discovery of biomarkers for psychiatric disorders and other diseases.