Despite some remarkable breakthroughs in the neurosciences, the human brain remains largely a mystery. Thanks to technological advances in the last few decades scientists have begun to unravel its secrets, and key discoveries promise to transform the treatment of neurological conditions. When it comes to fully comprehending the basic mechanisms of neuropsychiatric diseases such as schizophrenia and autism, however, there is still some way to go. A new approach now allows us to study the complex interplay of neurons in health and disease by observing reductionist replicas of the human brain, or ‘brain organoids’, miniature living compounds cultivated in the lab from human stem cells. Paola Arlotta, a professor at Harvard University, is a world-leading pioneer in this field, and her work lies at the intersection of neuroscience and stem cell biology. Paola’s main field of interest is the cerebral cortex, the part of the brain that makes us human and controls the most complex functions that we are capable of. Her brain organoids provide a novel platform for in vitro modelling of diseases and remedies. At Falling Walls, Paola talks about the fascination of watching brains grow under the microscope and the ethical responsibilities that come with this revolutionary method.