Urdd Gobaith Cymru, literally, the Welsh League of Hope, but normally translated as the Welsh League of Youth, or merely referred to as the Urdd, is a Welsh-medium youth movement with over 1,500 branches and over 50,000 members (as of 2006). The Urdd organises activities for its members ranging from small-scale meetings to the Urdd eisteddfod which lasts for a week, is attended by tens of thousands of visitors (over 100,000 in 2005) and is televised on S4C. The Urdd is often claimed to be the largest youth movement in Europe and the Urdd Eisteddfod the largest youth festival in Europe. The first claim is certainly mistaken. The Urdd has 50,000 members, while the Scouts in the UK alone, let alone Europe, have half a million. The Urdd itself, wisely, only claims to be Wales's largest youth organisation.

The Urdd was founded in 1922 by Sir Ifan ab Owen Edwards through an appeal in the children's magazine Cymru'r Plant ("the Children's Wales"). In 1925 the first "Peace and Goodwill Message" from the youth of Wales to the youth of the world was issued by the Urdd, a custom that continues every year on the 18th May. The logo of the Urdd is a triangle of green (symbolising Wales), red (symbolising fellow-man), and white (symbolising Christ): faithfulness to these three elements is the basis of the Urdd's motto. A version of this logo with eyes, arms and legs becomes the character Mistar Urdd.

There are three age groups within the Urdd: primary (school) members under 11, secondary (school) members between 11 and 16, and Urddaholics between 16 and 25. The Urdd is organised into local groups whose catchment areas are based on those of the Welsh-medium schools of Wales. These groups in turn are organised into larger county-based regions. Competitions organised by the Urdd include sports (including rugby union, swimming, darts, and pool amongst others); and arts, which are generally contested at eisteddfodau. Competitions begin at the local stage. Winners of local competitions go on to regional competitions; winners of these then progress to the national finals, which for non-sports competitions is generally the Urdd Eisteddfod. Depending on the intensity the school brings to the matter, school eisteddfodau and Urdd competitions can be a major aspect of being in school in Wales.

The Urdd has several residential centres, two of which began as campsites. The first to be established (in 1932) was in Llangrannog; another is Glan-llyn by Llyn Tegid; and a third is in the Millennium Centre in Cardiff. As well as trips to these centres, the Urdd organises trips abroad for its members.