Thermodynamics, a fundamental principle of physics, explores the interplay between heat, work, energy and temperature within systems. According to the laws of thermodynamics, energy is conserved (first law) and entropy tends to increase with increase in free energy (second law). This shapes the behaviour of both microscopic particles and macroscopic processes. Systems are classified as open, closed or isolated based on their interaction with surroundings, undergoing various thermodynamic processes like isothermal and adiabatic changes. Entropy, a measure of disorder, dictates the directionality of natural processes and informs the efficiency of engines, refrigerators and other engineered systems. Its principles validate modern science, providing essential tools for understanding and predicting how energy transforms and systems evolve in the physical world. This book discusses the fundamentals as well as modern approaches of thermodynamics. It presents the complex subject of thermodynamics in the most comprehensible and easy to understand language. This book will help new researchers by foregrounding their knowledge in this branch.