Nature Reviews Neuroscience, Published online: 15 July 2026; doi:10.1038/s41583-026-01065-x Spontaneous brain activity is experimentally unconstrained. Embracing this freedom unlocks rich opportunities beyond task-based and naturalistic paradigms, the traditional Herculean pillars of constrained cognition. Emerging data-driven approaches can extract cognitively meaningful and translationally relevant insights from spontaneous brain activity: across profoundly altered states, across the human lifespan and across species.