Maria Montessori (1870-1952), Italy's first female physician, developed her educational method through scientific observation of children at the Casa dei Bambini in Rome's San Lorenzo district in 1907. Her insight: children have an innate drive to learn and self-construct, given the right environment, the right materials, and adults who observe rather than direct.
Authentic Montessori is built on three pillars: a prepared environment (a beautiful, ordered classroom designed for child autonomy), specific materials (purpose-designed objects that isolate concepts and self-correct errors), and trained guides (teachers who observe children, present materials when developmentally ready, and otherwise step back). Children work individually or in small groups, choose their own work, and progress at their own pace.
The method runs through five planes of development identified by Montessori: 0-6 (the absorbent mind), 6-12 (the reasoning mind), 12-18 (the social being / Erdkinder), 18-24 (the specialised adult). Most international Montessori schools deliver the first two planes (Casa 3-6, Lower & Upper Elementary 6-12). Adolescent Montessori (12-18) is rarer but growing, with notable examples like Phuket International Academy in Thailand and Lake Country School in the US.
The Association Montessori Internationale (AMI), founded by Maria Montessori herself in 1929, is the gold-standard accreditor. AMI-trained guides complete a 1-year diploma after a master's degree. AMI-recognised schools commit to Montessori fidelity. Be aware: many schools call themselves "Montessori" without AMI accreditation, with widely varying quality. The international "Montessori-inspired" or "Montessori-themed" sector is much larger than the AMI authentic-method sector.