This book offers a unified approach to a challenging and far-reaching subject: thermodynamics. Spanning disciplines — from mechanical, chemical, and materials engineering to physics, chemistry, biology, and mathematics — it explores the field through the foundational concepts of energy, enthalpy, and entropy. These core principles are introduced early in the text and consistently reinforced through detailed explanations, intuitive reasoning, and fully worked examples that promote both conceptual understanding and practical insight.It complements standard course materials by offering additional clarity, depth, and a broader, interdisciplinary perspective. Special emphasis is placed on the analysis of thermal systems, including classical heat engines, refrigeration and air conditioning cycles, and often underexplored passive components such as throttles, diffusers, and nozzles.The book makes thermodynamics more accessible to a wider audience and encourages meaningful understanding across fields of study by presenting a methodical approach to mathematical derivations. Every algebraic step is presented in full, supporting the principle that deep understanding comes from mastering underlying concepts — not just formal manipulations.