We present some recent models describing the flow of mineral oils with a high content of paraffin at temperature below thecloud point(i.e. the crystallization temperature of paraffin) and more specifically below thepour pointat which the crystals aggregate and the oil takes a jel-like structure. From the rheological point of view such a system can be modelled as a Bingham fluid whose parameters evolve according to the volume fraction of crystallized paraffin and the aggregation degree of crystals. The problem is not studied in its full generality since a number of different situations can arise presenting various degrees of complexity. Therefore many questions remain open for further research.