Abstract: The invention generally concerns optical systems and, in particular, a method and device for joining at least one first and one second optical element to an optical composite element, as well as an optical composite element itself. In order to produce optical systems having at least two optical elements more easily and more economically, the invention provides a method for joining at least one first and one second optical element, in which the first optical element contains a first glass or a crystalline material, the second optical element contains a second glass, and the first glass or the crystalline material has a transformation temperature Tg1 or a melting temperature that differs from the transformation temperature Tg2 of the second glass, and at least the glass of the second optical element is heated and brought into contact with the glass or with the crystalline material of the first optical element.