"Foundations in Microeconomic Theory" is a scholarly book that honors the significant contributions of Hugo Sonnenschein to microeconomic theory. Compiled on the occasion of his 65th birthday, this work aims to showcase the impact of his mentorship on the academic community. The collection includes works from his students that have emerged over several decades. These contributions not only reflect the academic development of the authors but also the profound influence Sonnenschein has had on their research. The diversity of topics and approaches in the contributions highlights the breadth and depth of Sonnenschein's impact on economic theory and the training of economists. Each chapter is complemented by a brief introduction from the respective students, illuminating the context of the work and Sonnenschein's role in their academic journeys. Thus, this book is not only a tribute to an outstanding scholar but also a valuable compendium for anyone interested in microeconomic theory and the development of economists.