ABSTRACT Bipolar disorder (BD) is a devastating and poorly understood illness. Its burden exceeds $202 billion a year in direct treatment, societal and family costs. With no known cure and limited therapeutic options, 50% of patients attempt suicide at least once; up to 30% of patients do not respond to first-line treatment - even with the latest medications and psychosocial therapy - yet bipolar disorder has only received a small fraction of the amount of research attention that goes into serious non-psychiatric illness. Differentiating BD from major depression is crucial for understanding BD pathophysiology. Advances in imaging are beginning to offer the first detailed, reproducible, and reliable data on brain changes in BD and how they progress, but the lack of global initiatives in bipolar disorder has stalled research. The high cost of data collection - few studies scan more than a hundred patients - has led to underpowered studies whose findings often fail to replicate, cannot adequately model confounds, and often lack power to identify factors that modulate disease progression or recovery. Our ENIGMA Bipolar Initiative offers a new, cost-effective, innovative global approach - a new source of power to unblock this logjam by merging resources, capital infrastructure and talents of leading BD centers including data from 48 cohorts across the world. ENIGMA’s Global Alliance for Worldwide Imaging Genomics in Bipolar Disorder - builds on our thriving ENIGMA Consortium. ENIGMA’s approach merges data from tens of thousands of individuals and “gives us a power we have not had”, and is “breaking the logjam in neuroscience” (New York Times). The Lancet hailed ENIGMA as “Crowdsourcing meets Neuroscience”. In designing the ENIGMA-Bipolar initiative, we identified the most productive activities in the ENIGMA Bipolar working group, and organized them into 3 themes - imaging, genomics, and cross-disorder comparisons. This global initiative tackles key questions in BD: how does the illness affect the brain? What imaging and genomic biomarkers assist diagnosis, monitor treatment, and predict outcomes? How do BD genetic susceptibility loci affect the brain? With global data and expert teams from 15 countries (see Support Letters), we tackle imaging, genomic, and predictive questions about BD with unprecedented power. Across Brazil, Japan, the US, Canada, Norway, The Netherlands, Germany, France, Australia, and South Africa - what brain differences do we reproducibly detect in BD (with structural/diffusion MRI, connectomics and resting state fMRI)? How do they vary across life, with illness duration, by demographics, in women versus men, by age of onset, subtype and treatment? Using ENIGMA’s harmonized protocols, we will analyze the largest collection of multisite neuroimaging data in BD - diverse in age, ethnicity, treatment response - to track disease worldwide. In a new Cross-Disorder partnership of ENIGMA-BD and MDD, we use ENIGMA’s data-driven models to...