In developing the model for FA composition, two major obstacles were encountered. Firstly flows of FA cannot be modelled independently of other aspects of nutrition (e.g. protein deposition). Consequently, these aspects had to be addressed by a basic growth model that by itself may have a considerable influence on model predictions. A second obstacle was the virtual non-existence of balance data for poly-unsaturated FA deposition (e.g. linoleic and linolenic acid) or composition of de novo synthesized FA. Most studies seem to focus on the (relative) composition of selected tissues (mainly backfat), making mechanistic relations between dietary input and output very difficult. Consequently, it appears too ambitious to model accurately the FA composition of pigs, given the obstacles mentioned above. The development of this model has probably raised more questions (concerning lipid metabolism, FA partitioning, and tissue development) than answers. Nevertheless, the results of backfat FA composition were encouraging and should stimulate further development of the model.