This thesis examines the historical development and current state of amendment law in the Australian patent system. Initial research on modern amendment cases immediately showed that the confused, inconsistent and complex state of the law is a significant problem. There is a plethora of different analytical tools and legal tests being applied to assess an amendment, yet they were developed in a different area of patent law, that of fair basing. Such tools and tests are ill equipped to provide any real assistance to decision-makers faced with assessing an amendment. In fact, they seem to lead decision-makers away from applying the correct investigation as set out in the amendment provisions of the legislation. The thesis examines the history of amendment law so as to place its discussion of the current problems in context and provide a better understanding of why the problems arose. Four major events are discussed in the thesis. Together, these events have shaped Australian amendment law over the past century: (i) the development and introduction of the “substantially larger than or substantially different from” test into the British and Australian statutory amendment provisions; (ii) the development and introduction of the concept of fair basing into British and Australian patent law; (iii) the development and introduction of the modern British and Australian statutory test for amendments and the tiered amendment scheme; and (iv) the analogies drawn in modern British and Australian cases between fair basing and amendment that ultimately led to fair basing tests being cross-applied in Australia to assess the allowability of amendments. The thesis shows how the very harsh early British treatment of requests for amendment ultimately led to statutory change. It also locates, for the first time, the common law origins of the notion of “fair basing.” The 1949 British legislation implemented a new and different statutory test that was intended to liberalise the whole area of amendment law. It also added the requirement of “fair basing” into the legislation. However, the thesis shows that this last development occurred via well-intentioned legislators with a significant misunderstanding of patent law. The notion of fair basing injected a great deal of uncertainty into an area of law that was previously settled. Theoretically, and in practical application, it caused problems. Then, when decision-makers sought guidance on the new amendment provision, they applied the tests developed in fair basing cases to assess amendments, with the consequent deleterious effects. The Australian experience largely mirrored the British experience until 1977 when the British Act changed. The significance of the thesis is that it clearly demonstrates that the currently accepted dogma – that fair basing is equivalent to the “in substance disclosure” statutory test for amendments, so fair basing tests can be used to assess amendment – is unsound. The thesis isolates the problems inherent in the dogma and the examination of relevant case law confirms the main hypothesis that the current approach should be rejected. It simply operates to the prejudice of inventors, their competitors, the public and the patent system itself. Most importantly, the thesis shows that reform is urgently needed. Some possibilities for reform are suggested.