Kassel traces its history back to 913, when it was first mentioned as the place where King Conrad I singed two deeds, one of them certifying that Kassel had city rights. Since 1957 the city has served as a capital of Hesse-Kassel and a center of Calvinist Protestantism in Germany. In 1685 the city became a refuge for over 1,700 Huguenots, who found shelter in this newly established borough. In the nineteenth century the Brothers Grimm lived and wrote many of their fairy tales in Kassel. Shortly after the city became the capital of the Kingdom of Westphalia under one of Napoleon's brothers, Jerome. During World War II, over 90% of the city center was destroyed, over 10,000 people died and 150,000 lost their homes in the firestorm that can be compared only to that of Hamburg. In the post war years the city was rebuilt, still preserving some of its landmarks.

Due to the complete destruction in 1943, there are only few old buildings and monuments in Kassel, the oldest of these being the Druselturm, St. Martin Church and Bruderkirche preserving their medieval origin. The majority of historic buildings that have survived can be found outside the town center, including Wilhelmshone Palace constructed by Wilhelm IX of Hesse-Kassel in 1786. Today it is a museum, housing a significant collection of Greece-Roman antiques, world-renowned wall paper collection, and a gallery of paintings that comprises the second largest collection of Rembrandts in the country. Another famous attraction, the Oktagon is an enormous octagonal structure that carries a replica of Hercules Farnese. From this place down to Wilhelmshohe Palace runs a set of artificial cascades delighting guests during summer months. Constructed during the reign of Wilhelm IX, the Lions Castle is a sample of a medieval castle.

Kassel tourism showed signs of growth in the late nineteenth century when many travelers started coming to enjoy numerous cultural sights of the rebuilt city. Kassel is the scene of Documenta - an international exhibition of modern and contemporary art, and home to many world-famous museums and art galleries, namely Wilhelmshohe Palace, Museum fur Sepulkralkultur housing works by Rubens, Albrecht Durer, Rembrandt, Antoon Van Dyck and Franz Hals. Among other renowned sights is the first German observatory built in Kassel in 1558, and followed by the Ottoneum in 1604 and the first public museum in Europe in 1779. The latter was named Museum Fridericianum after its founder and by the nineteenth century held one of the world's largest collections of clocks and watches. Among the famous people who lived in Kassel are Jerome Bonaparte, the Brothers Grimm, Paul Reuter, Philipp Schneidemann and F.W. Murnau. The city is also the birthplace of renowned Annika Mehlhorn who completed in the 2004 Summer Olympics.

Kassel is twinned with many cities, including Florence, Italy (since 1952); Izmit, Turkey (since 1999); Mulhouse, France (since 1965); Rovaniemi, Finland (since 1972); Ramat Gan, Israel (since 1990); Yaroslavl, Russia (since 1988); and Arnstadt, Thuringia Germany (since 1989).