Some possible reasons why remediation is needed in mathematics instruction are presented. The need to diagnose the problem and then provide appropriate, effective remediation is discussed. Four guiding principles for diagnosis are presented, followed by six questions covering hypotheses about the source of the difficulty. Suggestions are then given for remedial work on errors with basic facts and on computational procedures. The game of "Multivision" is included. Finally, grouping for remediation is discussed. (MNS)