Overview of the data flow and main functional tasks of the HFI Data Processing Centre. Level 1 creates a database of the raw satellite data as a function of time. Level 2 builds a data model and produces sky maps at the six frequency of the instrument. This flow diagram illustrates the crucial role of the Instrument Model (IMO), which is both an input and an output of many tasks, and is updated iteratively during successive passes of the data. Level 3 takes these instrument-specific products and derives final astrophysical products. This paper is mostly concerned with Level 2 processing and its validation.