This article engages in a comparative analysis of espionage law in the UK and Australia to determine whether the laws in each country are effective and appropriate. It finds that, while the espionage laws in both countries are largely capable of effectively addressing modern espionage, this has come at the expense of appropriateness – specifically, aspects of the laws in both jurisdictions are complex, uncertain and overly broad, and defences and other safeguards for legitimate conduct have limitations. The article argues that, while the effectiveness of espionage (and other national security) laws is an important consideration, this must be balanced with appropriateness to ensure that core rule of law values and legal principles are not undermined.