This chapter uses the HKL package coordinate system to describe data algorithms and analysis. Data analysis makes specific assumptions which the collected data must, or at least should, satisfy. The description of data analysis and algorithms given here makes frequent references to the assumptions about the data and offers guidelines on how to make the experiment fulfil these assumptions. Topics covered include: diffraction from a perfect crystal lattice; autoindexing; coordinate systems; experimental assumptions; prediction of the diffraction pattern; detector diagnostics; multiplicative corrections (scaling); global refinement or post refinement; and graphical command centres. This chapter is also available as HTML from the International Tables Online site hosted by the IUCr.