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.