This thesis focuses upon recent Australian developments in the law of estoppel. It provides a justification and basis in principle for the doctrine of estoppel described in cases such as Waltons Stores (Interstate) Ltd v Maher (1987-1988) 164 C.L.R. 387 and Commonwealth of Australia v Verwayen (1990) 170 C.L.R. 394. This basis is found in the principle that we ought all to take reasonable steps to ensure the reliability of the assumptions that we induce in others. Ensuring the "reliability" of an induced assumption means ensuring that a party who relies upon the assumption does not thereby suffer harm: harm in the sense that he is worse off because the assumption has proved unjustified than he would have been had it never been induced. The thesis suggests a pattern for the development of the Australian law of estoppel reflecting that basis in principle. It further demonstrates the potential usefulness of the doctrine with specific reference to (i) pre-contractual negotiations and letters of intent, (ii) firm offers to contract, (iii) variations of contract unsupported by consideration, and (iv) the "battle of forms".