The line of Hesse-Kassel (or Hesse-Cassel) was founded by William IV, surnamed the Wise, eldest son of Philip the Magnanimous.

On his father's death in 1567, he received one half of Hesse, with Cassel as his capital; this formed the Landgraviate of Hesse-Kassel.

Maurice converted to Calvinism in 1605, became involved later in the Thirty Years' War, and, after being forced to cede some of his territories to the Darmstadt line, abdicated in 1627 in favour of his son William V (1602–1637).

His younger sons received apanages, which created several cadet lines of the house (Hesse-Rotenburg, Hesse-Eschwege and Hesse-Rheinfels), of which, with amalgamation, that of Hesse-Rheinfels-Rotenburg survived till 1834.

[2] The latter family died out in 1755, when William's grandson, Constantine (d. 1778), reunited the lands except Rheinfels, which had been acquired by Hesse-Kassel in 1735, and ruled them as Landgrave of Hesse-Rotenburg.