For the precise investigation of heat transfer mechanisms at surfaces, heat flux sensors of high sensitivity, low thermal resistance and small physical thickness are necessary. By optimizing the thermoelectric materials (Sb, Bi, Te), using microelectronic technology for a high package density and an optimized geometry for the sensors we have up to now attained an improvement in responsivity (signal per active area of the sensor) of a factor of 100 compared to the available conventional sensors. A further development to selective sensors for the separate but simultaneous measurement of different components of the heat transfer: convective, infrared and short wave has been realized. Sensors of up to 100 µV/(W/m2) sensitivity, consisting of a pair with selective coatings for the separate measurement of convective and radiative heat transfer are now available. A patent is pending.