Mr Murray-Jones is concerned to re-examin,e the rules which have developed with respect to the interpretation of s. 109 of the Constitution. He looks first at the “cover-the-field” test enunciated by Isaacs J. and its subsequent application by the High Court. He moves on to discuss a second test propounded by Dixon J. which would strike down State laws which altered, detracted or impaired the operation of a Commonwealth law. These tests are, of themselves, insufficient to meet the difficulties and a number of subsidiary tests for direct inconsistency, some of doubtful validity, are critically scrutinized. The position seems clear when the conclusion can be reached that the Commonwealth intended to provide a single, nation-wide code to regulate a particular area. When criminal proceedings for an offence which contravenes both State and Commonwealth law are considered, complications with respect to differing offences and penalties can frequently arise and the author concludes that the difficulties still require satisfactory resolution. One suggestion is that the CourtS' should more clearly articulate the various factors }vhich they take into account when reaching th,eir decisions.