Many measurements of ionospheric temperature have been made during the last decade and have shown a considerable departure from thermal equilibrium there. Theoretical work has led to a general understanding of the processes which determine the energy balance of the charged particles, and most features of the experimental results are well understood in the F-region. The position in the E-region where different methods of measurement, known to agree fairly well at greater heights, appear to disagree is less clear, whilst in the D-region no direct observations of charged particle temperature exist. The theoretical expectation here is that, on account of the rapid exchange of energy amongst the particles by collisional processes, temperature equality will prevail.