A dense deposit of misfolded protein that accumulates in tissues and disrupts organ function, most famously the amyloid-beta plaques that riddle the brains of people with Alzheimer’s disease. These plaques form when proteins that should remain soluble instead aggregate into insoluble fibrils, triggering inflammation and killing nearby cells. In Alzheimer’s, plaques cluster between neurons in memory centers, and their presence, now detectable with brain imaging, is a defining pathological feature, though their exact role in causing dementia remains debated. Similar plaques damage the heart in cardiac amyloidosis and the kidneys in renal amyloidosis. Research into clearing plaques with antibodies has produced the first disease-modifying Alzheimer’s drugs, making these microscopic deposits one of the most intensively studied structures in medicine.