Feeding herbivorous livestock is not only a current concern to the farmers, it is also constantly interfering with the problems raised by produce quality, cost saving, security and work simplification. Functional analysis, as an operational tool applied to the feeding systems, offers new prospects. Its theoretical and methodological bases are presented. The functional analysis of feeding systems is centred on how they work over the year ; special emphasis is laid on the securities the farmers provides for hazards, especially the climatic ones. An example is given of the analysis of feeding practices : a double sequence, on the one hand for the feed requirements corresponding to the farmer's objectives, on the other hand for the resources (grazed or distributed) mobilized, shows that the system is consistent over the year. The method can be utilized for the determination of the unchanging practices and the securities planned by the farmer, or for the finer analysis of a given year ; it makes it possible for instance to evaluate the capacity of a feeding system to integrate the constraints due to the management of the environment. Another possibility is the integration of considerations such as the planning of work on the farm or the working-out of technical results.