In the United States a preparatory school, or prep school, is usually a private secondary school (or high school) designed to prepare a student for higher education. In the United Kingdom and some parts of the Commonwealth of Nations, a Preparatory School, or Prep. School is a private school, designed to prepare a student for fee paying Public Schools. The age range catered for may be only eight- to thirteen-year-olds, or it may include younger pupils as well. A private school which only caters for under eights is a "pre-prep" and the junior departments of prep schools which cover the first years of schooling are also called "pre-prep".

Both forms have some things in common. Many are boarding schools; often, such schools are highly selective and academically challenging; they are largely independent of state or local authority control. Such schools are usually expensive, with high tuition used to pay excellent teachers and provide enriched learning environments, through features such as large libraries, advanced science laboratories, and new computers. British and New England preparatory schools, in particular, place a strong emphasis on sports.

In the United States, college preparatory schools often place an emphasis on offering a wide variety of Advanced Placement courses, rigorous academic programs, and small class sizes of 20 or less. Admission is rigorously selective and often based on standardized college entrance exams like the SAT.

The preparation for college that such courses provide, the higher-than-average SAT scores, and other attributes of their students lead many colleges and universities to actively recruit prep school students. In the United States, preparatory education is also often associated with the Preppy subculture.

Because they pay well (and/or offer superior working conditions), American college preparatory schools are often able to recruit faculty and staff with graduate degrees from the top universities, particularly the Ivy League. In contrast, most teachers at public schools usually hold only a bachelor's degree (which may not even be in the field that they teach) and a teaching certificate.

Partial list of international preparatory schools

Schule Schloss Salem (Salem, Germany)
Institut Alpin Videmanette (Rougemont, Switzerland )
Institute Le Rosey (Rolle, Switzerland )

Partial list of notable U.S. preparatory schools

The following is a partial list of preparatory schools in the United States:

• Atlanta International School (Atlanta, Georgia)
• Avon Old Farms School (Avon, Connecticut)
• Buckingham, Browne and Nichols (Cambridge, Massachusetts)
• Loomis Chaffee School (Windsor, Connecticut)
• Blair Academy (Blairstown, New Jersey)
• Brownell-Talbot School (Omaha, Nebraska)
• Choate Rosemary Hall (Wallingford, Connecticut)
• Cranbrook Kingswood (Bloomfield Hills, Michigan)
• The Buckley School (Los Angeles, California)
• The Collegiate School (New York City, New York)
• The Dalton School (New York City, New York)
• Deerfield Academy (Deerfield, Massachusetts)
• Emma Willard School (Troy, New York)
• Georgetown Visitation Preparatory School (Washington, DC)
• Governor Dummer Academy (Byfield, Massachusetts )
• Groton School (Groton, Massachusetts)
• Harvard-Westlake School (North Hollywood, California)
• The Hill School (Pottstown, Pennsylvania)
• Holderness School (Plymouth, New Hampshire)
• Holy Ghost Preparatory School (Bensalem, Pennsylvania)
• Hopkins School (New Haven, Connecticut)
• Horace Mann School (Riverdale, New York )
• Hotchkiss School (Lakeville, Connecticut )
• John Burroughs School (St. Louis, Missouri)
• Kent School (Kent, Connecticut)
• Lakeside School (Seattle, Washington)
• Lawrence Academy at Groton (Groton, Massachusetts)
• Lawrenceville School (Lawrenceville, New Jersey)
• MICDS (St. Louis, Missouri)
• Middlesex School (Concord, Massachusetts)
• Millbrook School (Millbrook, New York)
• Milton Academy (Milton, Massachusetts)
• Morristown-Beard School (Morristown, New Jersey)
• Noble and Greenough School (Dedham, Massachusetts)
• Northfield Mount Hermon (Northfield, Massachusetts)
• Peddie School (Hightstown, New Jersey)
• The Pembroke Hill School (Kansas City, Missouri)
• The Pingry School (Martinsville, New Jersey)
• Phillips Academy (Andover, Massachusetts)
• Phillips Exeter Academy (Exeter, New Hampshire)
• Rutgers Preparatory School (Somerset, New Jersey)
• St. Andrew's School (Middletown, Delaware)
• St. Ignatius College Preparatory (San Francisco, California)
• St. Mary's Preparatory (Orchard Lake, Michigan)
• St. Paul Academy and Summit School (St. Paul, Minnesota))
• St. Paul's School (Concord, New Hampshire)
• Sidwell Friends School (Washington, DC)
• Strake Jesuit College Preparatory (Houston, Texas)
• Tabor Academy (Marion, Massachusetts)
• The Thacher School (Ojai, California)
• The Walnut Hill School (Natick, Massachusetts)
• The Westminster Schools (Atlanta, Georgia)
• Western Reserve Academy (Hudson, Ohio)
• Williston Northampton School (Easthampton, Massachusett )
• Worcester Academy (Worcester, Massachusetts)
• The Kiski School (Saltsburg, Pennsylvania)

Partial list of British Preparatory Schools

• Bancroft's Preparatory School, Woodford Green
• Dragon School, Oxford
• Haslemere Preparatory School , Haslemere
• Fairfeild Preparatory School , Loughborough
• St Ives School, Haslemere

Partial list of Canadian preparatory schools

• St. Michael's College School (Toronto, Ontario)
• Upper Canada College (Toronto, Ontario)
• Trinity College School (Port Hope, Ontario)
• Appleby College (Oakville, Ontario)
• St. George's School (Vancouver, British Columbia)

In France, some high schools offer special postgraduate classes, equivalent in level to the first years of university, for students who wish to enter Grandes Écoles.