Irrigation in viticulture is the process of applying extra water in the cultivation of grapevines. It is considered both controversial and essential to wine production. In the physiology of the grapevine, the amount of available water affects photosynthesis and hence growth, as well as the development of grape berries. While climate and humidity play important roles, a typical grape vine needs 25-35 inches (635-890 millimeters) of water a year, occurring during the spring and summer months of the growing season, to avoid stress. A vine that does not receive the necessary amount of water will have its growth altered in a number of ways; some effects of water stress (particularly, smaller berry size and somewhat higher sugar content) are considered desirable by wine grape growers.

In many Old World wine regions, natural rainfall is considered the only source for water that will still allow the vineyard to maintain its terroir characteristics. The practice of irrigation is viewed by some critics as unduly manipulative with the potential for detrimental wine quality due to high yields that can be artificially increased with irrigation. It has been historically banned by the European Union's wine laws, though in recent years individual countries (such as Spain) have been loosening their regulations and France's wine governing body, the Institut National des Appellations d'Origine (INAO), has also been reviewing the issue.

In very dry climates that receive little rainfall, irrigation is considered essential to any viticultural prospects. Many New World wine regions such as Australia and California regularly practice irrigation in areas that couldn't otherwise support viticulture. Advances and research in these wine regions (as well as some Old World wine regions such as Israel), have shown that potential wine quality could increase in areas where irrigation is kept to a minimum and managed. The main principle behind this is controlled water stress, where the vine receives sufficient water during the budding and flowering period, but irrigation is then scaled back during the ripening period so that the vine then responds by funneling more of its limited resources into developing the grape clusters instead of excess foliage. If the vine receives too much water stress, then photosynthesis and other important processes such as nutrient storage could be impacted with the vine essentially shutting down. The availability of irrigation means that if drought conditions emerge, sufficient water can be provided for the plant so that the balance between water stress and development is kept to optimal levels.

The practice of irrigation has a long history in wine production. Archaeologists describe it as one of the oldest practices in viticulture, with irrigation canals discovered near vineyard sites in Armenia and Egypt dating back more than 2600 years. Irrigation was already widely practiced for other agricultural crops since around 5000 BC. It is possible that the knowledge of irrigation helped viticulture spread from these areas to other regions, due to the potential for the grapevine to grow in soils too infertile to support other food crops. A somewhat robust plant, the grapevine's largest need is for sufficient sunshine, and it is able to flourish with minimum needs of water and nutrients. In areas where its water needs are unfulfilled, the availability of irrigation meant that viticulture could still be supported.

In the 20th century, the expanding wine industries of California, Australia and Israel were greatly enhanced by advances in irrigation. With the development of more cost efficient and less labor-intensive ways of watering the vines, vast tracts of very sunny but dry lands were able to be converted into wine-growing regions. The ability to control the precise amount of water each vine received, allowed producers in these New World wine regions to develop styles of wines that could be fairly consistent each year regardless of normal vintage variation. This created a stark contrast to the Old World wine regions of Europe where vintage variation, including rainfall, had a pronounced effect on the potential wine style each year. Continuing research explored the way that controlled (or supplemental) irrigation could be used to increase potential wine quality by influencing how the grapevine responds to its environment and funnels resources into developing the sugars, acids and phenolic compounds that contribute to a wine's quality. This research lead to the development of ways to measure the amount of water retention in the soil, so that individual irrigation regimes could be plotted for each vineyard that maximized the benefits of water management.

The presence of water is essential for the survival of all plant life. In a grapevine, water acts as a universal solvent for many of the nutrients and minerals needed to carry out important physiological functions, and the vine receives these by absorbing the nutrient-containing water from the soil. In the absence of sufficient water in the soil, the root system of the vine may have difficulties absorbing these nutrients. Within the structure of the plant itself, water acts as a transport within the xylem to bring these nutrients to all ends of the plant. During the process of photosynthesis, water molecules combine with carbon derived from carbon dioxide to form glucose, which is the primary energy source of the vine, as well as oxygen as a by-product.

In addition to its use in photosynthesis, a vine's water supply is also depleted by the processes of evaporation and transpiration. In evaporation, heat (aided by wind and sunlight) causes water in the soil to evaporate and escape as vapor molecules. This process is inversely related to humidity with evaporation taking place at faster rates in areas with low relative humidity. In transpiration, this evaporation of water occurs directly in the vine, as water is released from the plant through the stomata that are located on the undersides of the leaves. This loss of water from the leaves is one of the driving factors that results in water being drawn up from the roots, and it also helps the vine combat against the effects of heat stress which can severely damage the physiological functions of the vine (somewhat similar to how perspiration works with humans and animals). The presence of adequate water in the vines can help keep the internal temperature of the leaf only a few degrees above the temperature of the surrounding air. However, if water is severely lacking then that internal temperature could jump nearly 18 °F (10 °C) warmer than the surrounding air which leads the vine to develop heat stress. The dual effects of evaporation and transpiration are called evapotranspiration. A typical vineyard in a hot, dry climate can lose as much as 1,700 U.S. gallons (6,400 L; 1,400 imp gal) of water per vine through evapotranspiration during the growing season.

There are essentially two main types of irrigation; primary irrigation, which is needed for areas (such as very dry climates) that lack sufficient rainfall for viticulture to even exist, and supplemental irrigation where irrigation is used to "fill in the gaps" of natural rainfall to bring water levels to better numbers as well as to serve as a preventive measure in case of seasonal drought conditions. In both cases, both the climate and the vineyard soils of the region will play an instrumental role in irrigation's use and effectiveness.