This paper considers the nature of the speech mechanism, and the effects on the speech spectrum of a high pressure helium-air environment. A comparison is made between the characteristics of speech distortions in a helium-air mixture and certain well-known characteristics of speech in normal air which give rise to similar effects.The criteria for good intelligibility are related to the performance of various helium speech unscrambling techniques which have been used. These unscrambling techniques are classified here into two main categories: those essentially using signal processing in the frequency domain and those using signal processing in the time domain. Consideration is also given towaveform coding techniques which involve a combination of both of these classes of signal processing.The ability of these categories to incorporate the various features required for good intelligibility in unscrambling helium speech is discussed, in order to highlight the potential importance of frequency domain approaches to future unscrambling developments.