Hirsau Abbey, formerly known as Hirschau Abbey, was once one of the most important Benedictine abbeys of Germany. It is located in the Hirsau borough of Calw on the northern slopes of the Black Forest mountain range, in the present-day state of Baden-Württemberg. In the 11th and 12th century, the monastery was a centre of the Cluniac Reforms, implemented as "Hirsau Reforms" in the German lands by William of Hirsau. The complex was devastated during the War of the Palatine Succession in 1692 and not rebuilt. The ruins served as a quarry for a period of time.

A Christian chapel at Hirsau dedicated to Saint Nazarius had already been erected in the late 8th century. The monastery itself was founded in about 830 by the Rhenish Franconian count Erlafried of Calw at the instigation of his son, Bishop Notting of Vercelli, who gave it the relics of Saint Aurelius of Riditio, an Armenian bishop who had died about 475, brought from Milan among other treasures.

They were first placed in the oratory of St. Nazarius' Chapel, while the monastery at Hirsau was being built on the count's estates. It was settled by a colony of fifteen monks descending from Fulda Abbey, disciples of Rabanus Maurus and Walafrid Strabo, under an abbot Liudebert or Lutpert. Count Erlafried endowed the new foundation with extended lands and other gifts, and made a solemn donation of the whole into the hands of Lutpert, on condition that the Rule of Saint Benedict should be observed. A first aisleless church, dedicated to Saint Aurelius, was not completed until 838, when it was consecrated by Archbishop Odgar of Mainz, who at the same time translated the relics from their temporary resting place to the new church.

Abbot Lutpert died in 853, having brought about a substantial increase both in the possessions of the abbey and in the number of the monks under his rule. Regular observance flourished under him and his successors and a successful monastic school was established. Over about a hundred and fifty years, under the care of the Counts of Calw, it enjoyed great prosperity, and became an important seat of learning. However, towards the end of the 10th century the ravages of pestilence, combined with the greed of its patrons and the laxity of the community, brought it to ruin. In 988 a severe plague devastated the neighbourhood and carried off sixty of the monks including the abbot, Hartfried. Only a dozen were left to elect a successor, and they divided into two parties. The more fervent chose one Conrad, whose election was confirmed by the Ruprecht, Bishop of Speyer, but some of the others, who favoured a more relaxed rule, elected an opposition abbot in the person of Eberhard, the cellarer. For some time the dispute ran high between the rival superiors and their respective followers. The Count of Calw supported the claims of Eberhard, but neither party would give way to the other and in the end the count brought in an armed force to settle the quarrel. The result was that the abbey was pillaged, the monks dispersed, and the valuable library destroyed. The count became master of the property and the abbey remained empty for over sixty years, during which time the buildings fell into a ruinous state.

In 1049 Pope Leo IX, uncle of Count Adalbert of Calw and grandson of the spoliator, came to Hirschau, and required Adalbert to restore the abbey. The count had the abbey church rebuilt in the style of a Roman basilica with an attached cloister. He renovated the premises, but so slowly that they were not refurbished until 1065, when the monastery was resettled by a dozen monks from the renowned Einsiedeln Abbey in Swabia, with Abbot Frederick at their head.

Abbot Frederick was deposed and the Counts of Calw dismissed his elected successor, Abbot Friedrich. They had a distant connection to Otto, Bishop of Regensburg. In May 1069, William, a monk of Saint Emmeram's Abbey was psent as successor. He immediately took over the management of the monastery, but refused to accept the abbatial benediction until after the death of his unjustly deposed predecessor in 1071. He was solemnly inaugurated by the Bishop of Speyer on Ascension Day in 1071.

When William came the condition of the monastery was far from satisfactory. The monks were living in cramped conditions, as the buildings were still incomplete and furthermore repeatedly affected by floods of the Nagold river. Count Adalbert still retained possession of some of the monastic property, together with a certain amount of unhelpful influence over the community, and regular discipline was very much relaxed. Abbot William's zeal and prudence by degrees remedied this unsatisfactory state of affairs and inaugurated a period of great prosperity, both spiritual and temporal. During the Investiture Controversy that shook the Holy Roman Empire, he secured the independence of the abbey from the Counts of Calw and placed its finances on a sound footing.

In 1089, monks from Hirsau founded Zwiefalten Abbey. William completed the buildings already begun and from 1082 afterwards greatly added to them, as the needs of the increasing community required, a new monastery complex on a high plateau on the opposite side of the Nagold river. The Sts Peter and Paul's abbey church, which was modelled on Cluny II finished about 981 under Abbot Majolus, was consecrated in 1091. The convent followed the next year, when it moved into the adjacent new monastic compound designed according to the Plan of Saint Gall, while old St Aurelius was converted into a priory. William also refounded the monastic school for which the abbey had formerly been famous throughout Germany.