Barrels were originally used as containers to store, mature and transport wine. Until fairly recently, most wines were shipped in barrels to wine merchants in Ireland and elsewhere and were only bottled on reaching their destination. Barrels were relatively inexpensive, easy to transport and watertight. The leftover casks were often then used to age whiskey.

The use of oak came about because of its unique qualities. It is watertight but slightly porous, allowing a gentle oxygenation of the wine and making it smoother. Oak adds flavour to wines, especially new oak, and sometimes a subtle texture too. A winemaker can choose oak barrels from a variety of forests. France and the US would be the most popular sources. The age, size, level of toast (see below) can make a huge difference to the finished wine.

A producer can choose French oak from Limousin, Tronçais, Vosges, Alliers or Nevers, all of which have a slightly different effect on the wine. Countries in central and eastern Europe also produce oak that is suitable for ageing wine. American oak is less expensive (a 300 litre barrel can cost up to €1,000, more for larger barrels) and gives different flavours. Producers can also decide the level of toast; high toast gives caramel, toasted nuts and chocolate; low toast spices and cedar.

In the past other trees were used to make barrels and some still are. Acacia is still favoured by some producers in Italy, Austria and Croatia. Chestnut was commonly used in Beaujolais and the Rhône. Eucalyptus, pine, ash, redwood, acacia and other woods were frequently used. However, most proved unsuitable either because they were difficult to work or they imparted flavours or too much tannin into the wine.