Topiary is the horticultural practice of training perennial plants by clipping the foliage and twigs of trees, shrubs and subshrubs to develop and maintain clearly defined shapes, whether geometric or fanciful. The term also refers to plants which have been shaped in this way. As an art form it is a type of living sculpture. The word derives from the Latin word for an ornamental landscape gardener, topiarius, a creator of topia or "places", a Greek word that Romans also applied to fictive indoor landscapes executed in fresco.

The plants used in topiary are evergreen, mostly woody, have small leaves or needles, produce dense foliage, and have compact or columnar (e.g., fastigiate) growth habits. Common species chosen for topiary include cultivars of European box (Buxus sempervirens), arborvitae (Thuja species), bay laurel (Laurus nobilis), holly (Ilex species), myrtle (Eugenia or Myrtus species), yew (Taxus species), and privet (Ligustrum species). Shaped wire cages are sometimes employed in modern topiary to guide untutored shears, but traditional topiary depends on patience and a steady hand; small-leaved ivy can be used to cover a cage and give the look of topiary in a few months. The hedge is a simple form of topiary used to create boundaries, walls or screens.

European topiary dates from Roman times. Pliny's Natural History and the epigram writer Martial both credit Gaius Matius Calvinus, in the circle of Julius Caesar, with introducing the first topiary to Roman gardens, and Pliny the Younger describes in a letter the elaborate figures of animals, inscriptions, cyphers and obelisks in clipped greens at his Tuscan villa (Epistle v.6, to Apollinaris). Within the atrium of a Roman house or villa, a place that had formerly been quite plain, the art of the topiarius produced a miniature landscape (topos) which might employ the art of stunting trees, also mentioned, disapprovingly, by Pliny (Historia Naturalis xii.6).

The clipping and shaping of shrubs and trees in China and Japan have been practised with equal rigor, but for different reasons. The goal is to achieve an artful expression of the "natural" form of venerably aged pines, given character by the forces of wind and weather. Their most concentrated expressions are in the related arts of Chinese penjing and Japanese bonsai.^{[citation needed]}

Japanese cloud-pruning is closest to the European art: the cloud-like forms of clipped growth are designed to be best appreciated after a fall of snow. Japanese Zen gardens (karesansui, dry rock gardens) make extensive use of Karikomi (a topiary technique of clipping shrubs and trees into large curved shapes or sculptures) and Hako-zukuri (shrubs clipped into boxes and straight lines).

In the 1550s and throughout Elizabeth I’s reign, topiary gardens grew in popularity. Knot gardens, formed from interlacing box hedges and aromatic herbs, featured square, symmetrical designs inspired by household patterns such as carpets and embroidery.

During the Italian Renaissance, gardens emphasized symmetry and proportion. The parterre developed from the knot garden, featuring clipped shrubs and statues arranged within geometric box-hedged patterns and became a central feature of formal European gardens. French designers expanded these ideas to grander scales, as seen in André Le Nôtre’s work at Versailles. Dutch gardeners became known for intricate geometric and figurative designs and exported their techniques across Europe. By this time, topiary appeared in both formal estates and cottage gardens.

Around 1578, Barnaby Googe noted that women were clipping rosemary "as in the fashion of a cart, a peacock, or such things as they fancy." In 1618 William Lawson suggested: