Herein, we discuss the in situ measurement of the electron temperature in the ionosphere/plasmasphere by means of DC Langmuir probes. Major instruments which have been reported are a conventional DC Langmuir probe, whose probe voltage is swept; a pulsed probe, which uses pulsed bias voltage; a rectification probe, which uses sinusoidal signal; and a resonance cone probe, which uses radio wave propagation. The content reviews past observations made with the instruments above. We also discuss technical factors that should be taken into account for reliable measurement, such as problems related to the contamination of electrodes and the satellite surface. Finally, we discuss research topics to be studied in the near future.

1. INTRODUCTION
 2. PAST OBSERVATIONS
  2.1 E region
  2.2 F region Ionosphere and Topside
  2.3 Plasmasphere
 3. ISSUES THAT MUST BE ADDRESSEDFOR PROPER MEASUREMENT
  3.1 Contamination of the Electrode Surface
  3.2 Surface Area of Satellite
  3.3 Stray Capacitance Effects and Others
 4. OBSERVATIONS FOR FUTURE SPACEMISSIONS
  4.2 F region and Top Side Ionosphere
  4.3 Plasmasphere
 5. TOWARD THE FUTURE
 ACKNOWLEDGEMENTS
 REFERENCES

• Koichiro Oyama(International Center for Space Weather Study and Education, Kyushu University,Institute of Plasma and Space Science, National Cheng Kung University) Corresponding Author