CIBSE Guide C was comprehensively updated for the previous edition in 2001 edition. Although basic physical data do not change with time, the refinement of measurement and calculation techniques and further research make regular review essential. Many of the changes to this edition are therefore small incremental changes, reflecting such refinement. It was however recognised that section 4, Flow of fluids in pipes and ducts, while heavily revised for the 2001 edition, was at that time unable to take account of the latest European research. The report of this research has now been obtained and its results distilled into this edition of Guide C. The opportunity has also been taken to rewrite and clarify the text and to delete many pages of tabular data providing pre-calculated pressure drops through pipes. It was felt that these tables have outlived their usefulness now that accurate pressure drops can easily be calculated using spreadsheets or computer programs.