The textbook "Observational Astrophysics" is aimed at physicists and graduate students and provides a comprehensive analysis of the fundamental and practical limitations that affect the performance of astronomical systems. The second edition has been completely revised and nearly doubled in size to enhance clarity and account for significant advancements over the past 15 years. The book covers both ground-based and space-based astronomy and their respective fields. It introduces the new generation of large ground-based telescopes and explains modern methods such as optical interferometry and adaptive optics. Additionally, it discusses ambitious concepts for future space missions and covers the entire electromagnetic spectrum, including the new astronomies made possible by gravitational waves and neutrinos.