In a little over ten years, organoid models – miniature, lab-grown clusters of cells that imitate real organs – have transformed how we study human development and disease while accelerating drug discovery. As a bonus, they’ve reduced our reliance on animal testing.Among these models, brain organoids – 3D, brain-like structures grown from stem cells – have progressed from simple cell clusters to sophisticated models that mimic important aspects of brain development and function. Recent breakthroughs have made them more complex: some organoids now show electrical activity similar to what is seen in a very early-stage human foetus. Others form networks of nerve cells that can send signals back and forth, in a similar way to how real brains communicate. These advances promise deeper insights into brain disorders such as Alzheimer’s and schizophrenia, and could revolutionise drug testing and personalised medicine. Yet, as complexity grows, so does ethical unease. Bioethicists warn that current rules for human and animal research don’t fit well for things that come from human cells, are alive, and are becoming more like real brainsUnder controlled conditions, brain organoids can self-organise into layers resembling – at a very basic level – the developing human brain. At first, organoids were basic models of the cortex. But newer methods now let scientists combine organoids that mimic different brain regions and even blood vessels, making them more complex and long-lasting. Researchers have also found ways to accelerate their development, enabling them to form working neural networks more quickly and even connect with robots.Organoids are useful because they let scientists study human biology without invasive procedures. They can show how the brain develops in early life – something we normally can’t see inside the womb. They can also mimic conditions like Alzheimer’s and autism so researchers can understand them better and test new treatments. Organoids provide safer, more reliable ways to test drugs and help reduce the need for animals in research, supporting global efforts to phase out mandatory animal testing.Yet, in many ways, organoids are not really miniature brains at all. They lack sensory input, body integration and do not replicate the vast complexity of a human brain. Human brain organoids. NIAID/Shutterstock.com, CC BY Still, as organoids build neural networks and show electrical activity like that seen in premature babies, an important question arises: when does this level of complexity suggest they might have some kind of feeling or experience? This is the main ethical concern. If organoids can process information or change their behaviour in response to it, do they gain moral status?Consciousness remains one of science’s most elusive concepts. There’s no clear definition or way to measure it. Some organoids have shown brain-like activity similar to that of premature babies, sparking headlines and public concern.Many scientists argue that these signals just show early, immature brain activity, not actual awareness. Still, ethical cautions suggest we should consider thresholds for how complex and active organoids become before proceeding unchecked.Regulatory limboEthical frameworks for research assume two categories: human subjects and animals. However, organoids fit neither. They are human-derived but not a person; living but not sentient. This ambiguity makes oversight of this field of research difficult. In the US and Europe, organoid research falls under general tissue-use regulations, focusing on donor consent rather than organoid welfare. China recently introduced the first comprehensive organoid guidelines, covering things like the possibility of consciousness and mixing human and animal cells. Most other countries lack clear rules. Because of this, experts are calling for international oversight, ongoing consent mechanisms and advisory panels to keep research transparent and maintain public trust.Brain organoids now sit at a crossroads: powerful enough to reshape neuroscience, yet complex enough to challenge our ethical comfort zones. As these models edge closer to behaviour we normally associate with living brains, the world needs clearer rules, shared standards and open dialogue.The science is moving fast, and our ethical frameworks must evolve just as quickly if we want this revolution to benefit society responsibly.James Poulter receives research funding from UK Research and Innovation.