Abstract: A thermal cycler for controlled heating and cooling of biological samples uses eight Thermoelectric Coolers (TECs), positioned between a metal sample plate and a heat sink (2). By altering the polarity of the applied current through the TEC, the temperature of the sample plate can be raised or lowered. When uniform heating and cooling of the sample plate is required, all eight TECs are connected in series and in the same polarity. To produce temperature gradients across the sample plate, the relative polarity of individual TECs is changed or they are bypassed altogether.