A temperature derivative sensing and regulating device and method is described which facilitates the maintainance of a very stable isothermal environment for an item, apparatus or facility or a constant rate of temperature change. The device comprises a block (6) of material having no internal heat sources, a thermal conductivity greater than 100 Wm K and a specific heat capacity greater than 400JK kg . The block (6) is surrounded by means for measuring the heat flow to and from the block (6) and the device is further provided with means for generating a signal proportional to said heat flow and means for interfacing with the controller of a heating or cooling means associated with the item, apparatus or facility.