Adult diffuse glioma are a diverse group of intracranial neoplasms associated with a disproportional large number of productive life years lost, thus imposing a highly emotional and significant financial burden on society. Patient death is the result of an aggressive course of disease following diagnosis. The Cancer Genome Atlas and similar projects have provided a comprehensive understanding of the somatic alterations and molecular subtypes of glioma at diagnosis. However, gliomas undergo significant molecular evolution during the malignant transformation. We review current knowledge on genomic, epigenomic and transcriptomic abnormalities before and after disease recurrence. We outline an effort to systemically catalogue the longitudinal changes in gliomas, the Glioma Longitudinal Analysis Consortium. The GLASS initiative will provide essential insights into the evolution of glioma towards a lethal phenotype with the potential to reveal targetable vulnerabilities, and ultimately, improved outcomes for a patient population in need.