The chapter introduces the concept of axions as a candidate for dark matter and discusses the cosmological implications of this idea. It begins by defining key concepts in cosmology, such as the Friedmann equations, the Hubble parameter, and the expansion of the universe. The chapter then delves into the thermodynamics of the early universe, explaining how particles and radiation behave during the radiation-dominated era. It highlights the importance of the comoving entropy density being conserved and how this affects the evolution of the universe. The chapter also reviews the history of the universe, from inflation to the electroweak symmetry breaking, and discusses the observational evidence for dark matter, including the cosmic microwave background (CMB) and the matter power spectrum. It emphasizes the significance of the CMB in providing a snapshot of the early universe and the constraints it places on the nature of dark matter. The focus then shifts to axions as a potential dark matter candidate, discussing the misalignment mechanism for producing axions and the conditions under which they would be created. The chapter explores the implications of axions being cold dark matter, including their mass, relic density, and the imprints they would leave in astrophysical data. It also touches on the experimental searches for axions, such as direct detection methods, and the challenges and prospects of detecting axions. Overall, the chapter provides a comprehensive overview of the current understanding of axions as dark matter, highlighting the theoretical and observational frameworks that support this idea and the ongoing efforts to confirm or refute it.The chapter introduces the concept of axions as a candidate for dark matter and discusses the cosmological implications of this idea. It begins by defining key concepts in cosmology, such as the Friedmann equations, the Hubble parameter, and the expansion of the universe. The chapter then delves into the thermodynamics of the early universe, explaining how particles and radiation behave during the radiation-dominated era. It highlights the importance of the comoving entropy density being conserved and how this affects the evolution of the universe. The chapter also reviews the history of the universe, from inflation to the electroweak symmetry breaking, and discusses the observational evidence for dark matter, including the cosmic microwave background (CMB) and the matter power spectrum. It emphasizes the significance of the CMB in providing a snapshot of the early universe and the constraints it places on the nature of dark matter. The focus then shifts to axions as a potential dark matter candidate, discussing the misalignment mechanism for producing axions and the conditions under which they would be created. The chapter explores the implications of axions being cold dark matter, including their mass, relic density, and the imprints they would leave in astrophysical data. It also touches on the experimental searches for axions, such as direct detection methods, and the challenges and prospects of detecting axions. Overall, the chapter provides a comprehensive overview of the current understanding of axions as dark matter, highlighting the theoretical and observational frameworks that support this idea and the ongoing efforts to confirm or refute it.