Decanting has a mystique that probably does more harm than good. If you have ever watched someone swirl wine around a beautiful crystal decanter for twenty minutes and wondered whether any of it actually matters, the honest answer is: sometimes yes, sometimes no, and the difference is knowable.
There are two distinct reasons to decant, and they apply to different situations.
Reason one: removing sediment
Older red wines - and some ports and Madeiras - develop a sediment in the bottle over time. This is a natural by-product of the wine's evolution and is not a flaw, but it is unpleasant in the glass. The sediment is bitter and gritty, and it makes the wine look cloudy.
To remove it, stand the bottle upright for at least a day before opening to allow the sediment to settle to the bottom. When you pour, go slowly, and stop when you see the sediment approaching the neck. Using a light source - a candle or a torch held below the bottle - lets you see the sediment coming and stop at the right moment. This kind of decanting is purely practical and there is no debate about whether it works.
Which wines need it? Anything over ten years old has a reasonable chance of having sediment. Bordeaux, northern Rhone reds, Barolo, and vintage port are the most common candidates. Young wines rarely throw sediment.
Reason two: aeration
This is the more contested territory. The theory is that exposing wine to oxygen opens it up, softening tannins and releasing aromas that would otherwise take years to develop in the bottle. The evidence supports this for some wines and not for others.
Big, structured reds - young Barolos, tannic Bordeaux, Ribera del Duero - can benefit significantly from an hour or two in a decanter before serving. These are wines built for the long term, and opening them young without aeration can make them feel closed, hard, and less enjoyable than they will eventually become. A couple of hours' breathing in a wide decanter gives them something of the effect that years in bottle would otherwise produce.
A simpler approach that also works: open the bottle an hour before serving and let it breathe in the bottle. The surface area of the wine exposed to air is smaller than in a decanter, so the effect is gentler, but for moderately structured wines it is usually sufficient.
When not to bother
Light reds - Burgundy, Beaujolais, Loire Cabernet Franc - generally do not improve with prolonged aeration and can even lose their charm if left too long. The delicate aromas that make a good Pinot Noir interesting can dissipate with exposure to air rather than develop. Fifteen minutes in a decanter may help; two hours will not.
White wines rarely need decanting, though very old, complex whites - aged white Burgundy, for instance, or old-vine Chenin Blanc - can benefit from a short spell in a decanter to open up. Young whites, light whites, and sparkling wines should never be decanted.
The practical rule
Decant when you know the wine is young and structured, or when you know it is old enough to have thrown sediment. Do not bother for everyday bottles or for wines where delicacy is the point. And if you are unsure, opening the bottle an hour early costs you nothing and often helps.
The decanter itself is not magic. It is just a vessel with a wide base that lets the wine breathe. A clean jug does the same job. Use whatever is convenient and does not make you feel self-conscious at the table.