Assists in automating the testing of terms in mixed models when 'asreml' is used to fit the models. The content falls into the following natural groupings: (i) Data, (ii) Object manipulation functions, (iii) Model modification functions, (iv) Model testing functions, (v) Model diagnostics functions, (vi) Prediction production and presentation functions, (vii) Response transformation functions, and (viii) Miscellaneous functions. A history of the fitting of a sequence of models is kept in a data frame. Procedures are available for choosing models that conform to the hierarchy or marginality principle and for displaying predictions for significant terms in tables and graphs. The package 'asreml' provides a computationally efficient algorithm for fitting mixed models using Residual Maximum Likelihood. It can be purchased from 'VSNi' < http://www.vsni.co.uk/> as 'asreml-R', who will supply a zip file for local installation/updating.