Wacken Open Air (stylised W:O:A) is an open-air heavy metal music festival. It takes place annually in summer in the village of Wacken in Schleswig-Holstein, northern Germany, 80 kilometres (50 mi) northwest of Hamburg.

W:O:A is usually held at the beginning of August and lasts four days. The festival was first held in 1990 as a small event with about 800 visitors and six local German bands. It is currently considered the biggest heavy-metal festival in the world. In 2011, the festival attracted 80,000 festival visitors and 6,000 personnel for a total of 86,000 attendees.

The festival traditionally ends on the first Sunday in August, and at midnight the following Monday tickets go on sale for the next year. Remarkably, all 75,000 tickets were sold out within 48 hours for 2014, 12 hours for 2015, and 23 hours for 2016, despite the fact that the lineup (with the exception of rumors or headliners) had not been announced. Including 2019, the festival was sold out fifteen times in a row. The non-optional basic ticket price for all four days, including camping for a week, was €220 in 2018 and for 2019 as well. In 2018, 197 bands were playing on nine stages.

The international significance of the festival is shown by the attendees in recent years consisting of 30% foreigners, with up to 10% non-Europeans, from about 30–40 different countries all around the world. Many metal fans travel from half a world away to stand in cow meadows before stages set in the middle of nowhere. In 2017, an official count confirmed visitors from over 80 different nations at the festival. A lot of metalheads, metal and hard rock bands worldwide are keen to make the journey to «The Mecca of Heavy Metal Culture» (Sam Dunn / Metal: A Headbanger’s Journey ), «The Holy Land» (Wacken 3D – a 2014 movie shown in Germany, Switzerland, Austria, and Italy), «The Cathedral of Heavy Metal» (Paul O’Neill / Trans-Siberian Orchestra), or «The Summit of Heavy Metal» (Thomas Gabriel Fischer / Celtic Frost) at least once in their lifetime, just for the experience.