This book constitutes the refereed proceedings of the 19th International Symposium on Theoretical Aspects of Software Engineering, TASE 2025, held in Limassol, Cyprus, during July 14–16, 2025. The 20 full papers, 1 short paper, 2 invited papers were carefully selected from 66 submissions. The papers present the latest developments in formal and theoretical software engineering methods and techniques. They are grouped in topical sections on program verification; verification and concurrency; SAT and SMT solving; trustworthy AI and system software; program analysis using machine learning; security; and dynamic analysis.