The Montessori method is both an educational philosophy and a methodology. It was originally developed in the late 19th century by Italian educator Maria Montessori. Many Montessori schools are preschool or elementary school in level, but there are some Montessori programs that begin with infants or end at 12th grade.

Concepts
The Montessori philosophy is built upon the idea that children develop and think differently than adults; that they are not merely "adults in small bodies".

The Montessori method discourages traditional measurements of achievement (grades, tests) under the premise that it is damaging to the inner growth of children (and adults). Feedback and qualitative analysis of a child's performance does exist but is usually provided in the form of a list of skills, activities and critical points, and sometimes a narrative of the child's achievements, strengths and weaknesses, with emphasis on the improvement of those weaknesses.

The method was developed from observations of young children from which a set of universal characteristics of children was created for each level of development. The Montessori method has two primary development levels. The first level is birth through six years old, the second level is ages 6-12. A Montessori classroom for the first level is called the casa dei bambini ("children's house"), and focuses on individually-paced learning and development. In the second level, working with others is encouraged, and "cosmic education" is introduced.

As an educational approach, the Montessori method's focus is on the individuality of each child, respecting their needs or talents as opposed to the needs of the class as a whole. A goal is to help the child maintain his or her natural joy of learning. Montessori believed that children had a cosmic embryo that would naturally grow as the child matures.

The Montessori method encourages independence and freedom with limits and responsibility. The youngest children are guided in practical life skills, e.g., domestic skills and manners. These skills are emphasized with the goal of increasing attention spans, hand-eye coordination, and tenacity. The Montessori Method states that satisfaction, contentment and joy result from the child feeling like a full participant in daily activities. Montessori education carried through the elementary and high school years follows the child's emerging tendency for peer-oriented interactions and still emphasizes that each student is the guardian of his or her own intellectual development.

Premises
* A view of children as competent beings capable of self-directed learning.
* That children learn in a distinctly different way from adults.
* The ultimate importance of observation of the child interacting with her or his environment as the basis for ongoing curriculum development. Presentation of subsequent exercises for skill development and information accumulation are based on the teacher's observation that the child has mastered the current exercise(s).
* Delineation of sensitive periods of development, during which a child's mind is particularly open to learning specific skills or knowledge, including language development, sensorial experimentation and refinement, and various levels of social interaction.
* A belief in the "absorbent mind", that children from birth to around age 6 possess limitless motivation to achieve competence within their environment and to perfect skills and understandings. This phenomenon is characterized by the young child's capacity for repetition of activities within sensitive period categories, such as exhaustive babbling as language practice leading to language competence.
* That children are masters of their environment, which has been specifically prepared for them to be academic, comfortable, and allow a maximum amount of independence.
* That children learn through discovery, so didactic materials that are self-correcting are used as much as possible.

Implementation
Montessori is a highly hands-on approach to learning. It encourages children to develop their observation skills by doing many types of activities. These activities include use of the five senses, kinetic movement, spatial refinement, small and large motor skill coordination, and concrete knowledge that leads to later abstraction.

Classrooms
Montessori classrooms are child centric. Furniture is child-sized, where no teacher's desk is present. The typical classroom consists of four areas: Practical Life, Sensorial, Language, and Mathematics. Practical Life includes activities such as buttoning, sweeping, pouring, slicing, tying, etc. Sensorial includes activities to stimulate and train hearing, touch, smell, and taste.

Most Montessori classrooms try to include ways for the children to interact with the natural world, perhaps through a classroom pet (rabbits, gerbils, mice, etc.) or a small garden where the children can plant vegetables or flowers.

In schools that extend to the upper grades, each Montessori classroom still includes an approximately three-year age range. This system allows flexibility in learning pace and allowing older children to become teachers by sharing what they have learned. The intent is to establish a non-competitive atmosphere in the classroom. The belief is that class work which is different for each child results in students who are less likely to try to follow where other children are academically.

Materials
Every activity has its place in the classroom and is self-contained and self-correcting. The original didactic materials are specific in design, conforming to exact dimensions, and each activity is designed to focus on a single skill, concept, or exercise. All of the material is based on SI units of measurement (for instance, the Pink Tower is based on the 1cm cube) which allows all the materials to work together and complement each other, as well as introduce the SI units through concrete example. In addition to this, material is intended for multiple uses at the primary level. A perfect example of this is the "Knobbed Cylinder" materials. Not only do they directly offer a sensorial lesson, but indirectly the child's grip on the cylinders paves the way for holding a pencil, and the grades of cylinders allow for an introduction to mathematics.

Other materials are often constructed by the teacher: felt storyboard characters, letter boxes (small containers of objects that all start with the same letter) for the language area, science materials (e.g. dinosaurs for tracing, etc.), scent or taste activities, and so on. The practical life area materials are almost always put together by the teacher. (All activities, however, must be neat, clean, attractive and preferably made of natural materials such as glass or wood, rather than plastic. Sponges, brooms ,and dustpans are provided and any mishaps (including broken glassware) are not punished but rather treated simply as an opportunity for the children to demonstrate responsibility by cleaning up after themselves.) At higher grade levels, the teacher becomes more involved in creating materials since not only the students' capacities but also the potential subjects widen considerably. Many of the earlier materials, moreover, can be revisited with a new explanation, emphasis, or use; for example, the cube that a five-year-old used as an exercise in color matching is revealed to the elementary level student to physically embody the mathematical relationship (a+b)3=a3 + 3a2b + 3ab2 +b3.

Lessons
A child does not engage in an activity until the teacher or another student has directly demonstrated its proper use, and then the child may use it as desired (limited only by individual imagination or the material's potentially dangerous qualities). Each activity leads directly to a new level of learning or concept. When a child actively learns, that child acquires the basis for later concepts. Additionally, repetition of activities is considered an integral part of this learning process, and children are allowed to repeat activities as often as they wish. If a child expresses boredom on account of this repetition, then the child is considered to be ready for the next level of learning.

The child proceeds at his or her own pace from concrete objects and tactile experiences to abstract thinking, writing, reading, science, and mathematics. In the language area, for instance, the child begins with the sandpaper letters (26 flat wooden panels, each with a single letter of the alphabet cut from sandpaper and affixed to it). The child's first lesson is to trace the shape of the letter with their fingers while they say the phonic sound of the letter. A possible next level activity would then be the letter boxes (small containers each with a letter on the top, filled with objects that begin with that letter). After mastering these, the child may move on to the word boxes (small containers each with a short three-letter word on the top, for example, "CAT", containing a small wooden cat and the letters C, A, T). One child might move through all three levels of lessons in a few weeks while another might take several months; however, while there is a prescribed sequence of activities, there is no prescribed timetable. A Montessori teacher or instructor observes each child and provides each with their correspondingly appropriate lessons as they are deemed ready for them.

Montessori schools
There are currently over 8,000 privately held Montessori schools in the United States, as well as several hundred public schools that include Montessori programs (see below). Most schools have a primary program (from 3-6 years) and often a lower elementary (6-9 years). Upper elementary programs (9-12 years) are less common, although about one school in eight will have this program. At this time Montessori junior highs and high schools are rare. However, the first public Montessori high school in the country, Clark Montessori located in Cincinnati, Ohio, was started in 1994. Several pilot Montessori junior high schools, have opened based on writings by Montessori on Erdkinder, German for "Children of the World", which was a term Montessori coined for children ages 12 through 18. The last few years have seen the advent of infant and toddler Montessori programs. Many schools, offer "Mother and Child" programs in which parents can learn about Montessori and how to apply the philosophy to their child-rearing practices. In many other schools, the demand for high-quality childcare has spurred the growth of Montessori infant, or "Nido" (the Italian word for "nest") and toddler, or "Infant Community" programs.