Michele Jamiolkowski was deeply interested in many topics in geotechnical engineering. The link between his many contributions to the discipline was his deep appreciation for the need to develop techniques for predicting the performance of real structures. This focus on real structures was pervasive in all of his research, and led to his attention to experimental investigation of natural soils. Jamiolkowski recognized the importance of in situ tests to site characterization of natural soils, and consequently this paper focuses on this topic. Jamiolkowski was involved in many challenging projects, and he always considered each project as an occasion to improve the state of the art, to develop novel approaches in site characterization, to develop new in situ test interpretation methods, and to obtain quality experimental data. The paper summarizes major improvements to the state of the art that resulted from his contributions, as well lessons learned from major and iconic projects in which Jamiolkowski was involved.