CRDS is a package used for working with astronomical reference files for the HST and JWST telescopes. CRDS is useful for performing various operations on reference files or reference file assignment rules. CRDS is used to assign, check, and compare reference files and rules, and also to predict those datasets which should potentially be reprocessed due to changes in reference files or assignment rules. CRDS has versioned rules which define the assignment of references for each type and instrument configuration. CRDS has web sites corresponding to each project http://hst-crds.stsci.edu or https://jwst-crds.stsci.edu/ which record information about reference files and provide related services.