A set of recent experimental results obtained by resistivity measurements on the process of formation of G.-P. zones in Al-rich alloys (pre-precipitation phenomenon), suggests the opportunity of defining a particular transformation in a supersaturated solid solution, which for its special characteristics can be called " ideal pre-precipitation ". Ideal pre-precipitation is a process during which, as zones grow in size, their number and their internal concentration is changing continuously in such a way to give a partial minimum in the free energy of the system. In the present work, after a definition of ideal pre-precipitation, the importance of the model, its consequencies and the conditions for practical observation of ideal pre-precipitation are discussed. The concept of ideal pre-precipitation appears to be a very useful basis for the interpretation of pre-precipitation in real alloys ; some recent results show, for example, that in Al-10 % Zn alloy the pre-precipitation process seems to follow the ideal pattern