Castile and Len (UK: , US: ; Spanish: Castilla y Len [kastia i leon] (listen); Leonese: Castiella y Llin [kastjea i io]; Galician: Castela e Len [kastl leo]) is an autonomous community in north-western Spain.

It was created in 1983. Formed by the provinces of vila, Burgos, Len, Palencia, Salamanca, Segovia, Soria, Valladolid and Zamora, it is the largest autonomous community in Spain in terms of area, covering 94,222 km2. It is however sparsely populated, with a population density below 30/km2. While a capital has not been explicitly declared, the seats of the executive and legislative powers are set in Valladolid by law and for all purposes that city (also the most populated municipality) serves as de facto regional capital.

Castile and Len is a landlocked region, bordered by Portugal as well as by the Spanish autonomous communities of Galicia, Asturias, Cantabria, the Basque Country, La Rioja, Aragon, CastillaLa Mancha, the Community of Madrid and Extremadura. Chiefly comprising the northern half of the Inner Plateau, it is surrounded by mountain barriers (the Picos de Europa to the North, the Sistema Central to the South and the Sistema Ibrico to the East) and it is drained by the Douro River, flowing west toward the Atlantic Ocean.

The region contains eight World Heritage Sites. UNESCO recognizes the Cortes of Len of 1188 as the cradle of worldwide parliamentarism.

Alfonso VII (1 March 1105 – 21 August 1157), called the Emperor (el Emperador), became the King of Galicia in 1111 and King of León and Castile in 1126. Alfonso, born Alfonso Raimúndez, first used the title Emperor of All Spain, alongside his mother Urraca, once she vested him with the direct rule of Toledo in 1116. Alfonso later held another investiture in 1135 in a grand ceremony reasserting his claims to the imperial title. He was the son of Urraca of León and Raymond of Burgundy, the first of the House of Ivrea to rule in the Iberian peninsula.

Alfonso was a dignified and somewhat enigmatic figure. His rule was characterised by the renewed supremacy of the western kingdoms of Christian Iberia over the eastern (Navarre and Aragón) after the reign of Alfonso the Battler. Though he sought to make the imperial title meaningful in practice to both Christian and Muslim populations, his hegemonic intentions never saw fruition. During his tenure, Portugal became de facto independent, in 1128, and was recognized as de jure independent, in 1143. He was a patron of poets, including, probably, the troubadour Marcabru.