A novel method of guided wave-based structural health monitoring is developed in which no direct baseline data are required to identify structural damage. Conventional wave propagation structural health monitoring techniques involve the comparison of structural response data to a prerecorded baseline or reference measurement taken while the structure is in pristine condition. The need to compare new data to a prerecorded baseline can present several complications, including data management issues and difficulty in accommodating the effects of varying environmental and operational conditions on the data. To address the complications associated with baseline comparison, this new method accomplishes reference-free damage detection by acquiring what is referred to as an instantaneous baseline measurement for analysis. The instantaneous baseline technique is validated through both analytical and experimental testing. Analytical tests show that the instantaneous baseline method is able to correctly identify simulated damage. It is found experimentally that nonpermanent damage in the form of removable putty as well as permanent damage in the form of corrosion and cuts are all identifiable in thin aluminum plate test structures without direct comparison to baseline data when implementing the instantaneous baseline method.