The North Atlantic Oscillation (NAO) is the most important pattern of climate variability for the weather and climate of western Europe and the Mediterranean basin. It is defined by the difference in atmospheric pressure between the Icelandic Low (semi-permanent low-pressure centre) and the Azores High. The NAO index can be positive or negative, and each phase produces very different climatic effects. In the positive phase (NAO+), the pressure difference is greater than normal: westerly winds are strong, Atlantic storms track toward northern Europe bringing rain and mild conditions to Scandinavia and the British Isles, while the Mediterranean and Spain remain under anticyclonic influence with dry, mild winters. In the negative phase (NAO-), the pressure difference is weak: the jet stream shifts southward, allowing storms to cross the Iberian Peninsula and the Mediterranean, producing rainy winters in Spain and Portugal, and colder conditions across western Europe. The NAO decisively influences water resources across southern Europe: strongly positive NAO years coincide with winter droughts, while negative NAO years tend to replenish reservoirs and aquifers. Along with El Niño/La Niña, the NAO is a key predictor for seasonal precipitation outlooks. It has no regular periodicity, making long-term prediction challenging.