The Huygens thermal control concept was developed in the B1 and B2 study phases between Jan. 91 and Feb. 92. This paper describes the actual thermal design and mass budget. It also covers the most important trade-offs, concept modifications and parameter studies performed prior to the actual design.Natural convection tests on two boxes having the typical shape of Huygens boxes and conductivity tests with Basotect foam around a standing and a hanging box were made in 1991 at MBB. The foam keeps the descent module sufficiently warm during the descent phase to Titan's surface. The paper describes these tests, presents the test evaluations and compares measured and calculated convection rates and conductivities. Special tests for the selection, treatment and placement of the foam are described.