The discussion defines discretion, outlines the reasons for the existence of discretion, and examines the methods for controlling discretion. It also considers the evidential and procedural discretions recorded in cases, laws, and reports of law reform bodies and lists the known principles by which these discretions should be exercised. A concluding section explains how uses of discretion can be challenged on the basis of jurisdiction, errors of fact or law that prejudiced the defense, errors of principle, the merits of a decision, the legality of a decision, and other factors. Footnotes containing references to judicial decisions and laws in Australia, Canada, and New Zealand and index, table of statutes, and table of cases.