Bunch compactness (or density) is a grapevine specific trait that affects the commercial quality and sanitary status of wine and tablegrapes. Compact bunches are more susceptible to diverse pests and diseases such as Botrytis bunch rot and their berries ripen more heterogeneously, causing important economic losses through a reduction in crop yield and grape and wine quality. Bunch compactness is determined by the fraction of the morphological volume of the bunch that is filled by berries, but this simple definition contrasts sharply with the difficulty to measure it. While there are several objective and quantitative methods available to measure bunch compactness, the lack of a consistent approach between researchers makes comparing measurements difficult. The complexity of bunch compactness initially arises from the several bunch and berry traits that influence it, and from the distinct effects that these traits may cause in different cultivars. In addition to this genetic effect, diverse environmental signals impact on bunch compactness by affecting those primary factors that contribute to the solid component of the bunch (berry number, berry size) or to its spatial arrangement (rachis architecture). Last, several viticultural approaches, including agronomic techniques and growth regulators, have also proven to affect bunch compactness in different ways. This review aims to discuss present knowledge about this relevant grapevine trait.