This commemorative volume celebrates 50 years of Henryk Petryk's scientific career and his distinguished contributions to the theory of plastic instability, a critical area in materials science and mechanical engineering. His thirteen selected individual works are preceded by a new unifying introduction, which creates a coherent monograph. Its core consists of the first three extensive chapters—originally published in the prestigious Springer series CISM Courses and Lectures—and the most recent final chapter. Curated to showcase the evolution and enduring relevance of his pioneering research, this collection offers both a historical perspective and authoritative insights into one of the field’s most challenging topics.