The Arnold Arboretum is one of the world's finest research arboretums. It is 256 acres (107 hectares) in size, run by Harvard College, located in Jamaica Plain, Massachusetts, and open to the public. The Arboretum is very popular with many Boston residents, and most especially during the blossoming of its famous lilacs.

The Arboretum was established in 1872 by the will of James Arnold (1781-1868), a whaling merchant of New Bedford, Massachusetts, who gave a legacy "for the establishment and support of an arboretum, to be known as the Arnold Arboretum, which shall contain, as far as practicable, all the trees [and] shrubs . . . either indigenous or exotic, which can be raised in the open air". Prof. Francis Parkman of Harvard led the Arboretum for its first few months, and Charles Sprague Sargent (1841-1927) was appointed the Arboretum's first director in 1873. Sargent came to agreement with the City of Boston, in 1882, so that the Harvard-owned land on which the Arnold Arboretum was established became part of the city park system, but control of the collections rested with the Arboretum staff. The Arboretum agreed to keep the grounds open to the general public, free of charge, from sunrise to sunset every day of the year. As a result of this unique arrangement the Arboretum became part of the famous "Emerald Necklace", the 7 mile (11 km) long network of parks and parkways that Frederick Law Olmsted laid out for the Boston Parks Department between 1878 and 1892.

The Arboretum's design came from close collaboration with Olmsted, who laid out the path and roadway system and designated areas within the Arboretum for specific groups of plants. Early on, Sargent decided to arrange the plant collections by family and genus, following the then generally accepted classification system of Bentham and Hooker. He also developed a comprehensive library devoted to botany, horticulture, and dendrology, an equally notable herbarium to serve as the repository of specimens of woody plants from throughout the world, and a publication program that included both scholarly and semi-popular works. In addition, the Arboretum's exploration around the world, especially in eastern Asia, has brought many new plants into cultivation and greatly expanded our knowledge of their evolution and systematics. The Arboretum continues to maintain its living collections in the naturalistic style originally established by Sargent and Olmsted; collections are still arranged in large part according to the Bentham and Hooker classification system. Plant exploration also continues, with seven major collecting trips to eastern Asia sponsored by the Arboretum since 1977.

In January 2000, the living collections consisted of some 7,082 accessioned plants representing 4,544 botanical and horticultural taxa, with particular emphasis on the woody species of North America and eastern Asia and an especially comprehensive representation of Fagus (beech), Lonicera (honeysuckle), Magnolia, Malus (crabapple), Quercus (oak), Rhododendron, and Syringa (lilac). Collections of historical interest include the plants introduced from eastern Asia by Sargent, Ernest Henry Wilson , William Purdom , and Joseph Rock. In addition to its living collections the Arboretum holds a herbarium collection in excess of 5 million specimens, and over 40,000 volumes in its library.