The cosmological constant, denoted by Λ, is a fundamental concept in cosmology that represents the energy density of the vacuum. Recent observations suggest that Λ is positive and has a magnitude of approximately 10⁻¹²³ in natural units. This review discusses the cosmological constant from both cosmological and field theoretical perspectives, covering its kinematics, dynamics, observational evidence, and theoretical models. The review also explores alternative interpretations of the cosmological constant, relaxation mechanisms, the geometrical structure of de Sitter spacetime, thermodynamics of the de Sitter universe, and the role of string theory in the cosmological constant problem. The cosmological constant is a key component in the Friedmann equations, which describe the expansion of the universe. Observational evidence for a non-zero cosmological constant includes supernova results, the age of the universe, gravitational lensing, and CMBR anisotropies. Theoretical models such as quintessence and tachyonic scalar fields are discussed, along with the cosmic degeneracies they introduce. The review also addresses the problem of why the cosmological constant is so small compared to other energy scales in the universe, a challenge known as the cosmological constant problem. This problem is further complicated by the fact that the observed value of the cosmological constant is extremely fine-tuned, leading to the question of why it is positive and of such a small magnitude. The review highlights the importance of understanding the cosmological constant in the context of dark energy and its role in the accelerated expansion of the universe. It also discusses the implications of the cosmological constant for the structure formation in the universe and the constraints on dark energy from CMBR anisotropies. The review concludes with a discussion of the role of string theory in addressing the cosmological constant problem, emphasizing the need for a deeper understanding of the fundamental principles that govern the universe.The cosmological constant, denoted by Λ, is a fundamental concept in cosmology that represents the energy density of the vacuum. Recent observations suggest that Λ is positive and has a magnitude of approximately 10⁻¹²³ in natural units. This review discusses the cosmological constant from both cosmological and field theoretical perspectives, covering its kinematics, dynamics, observational evidence, and theoretical models. The review also explores alternative interpretations of the cosmological constant, relaxation mechanisms, the geometrical structure of de Sitter spacetime, thermodynamics of the de Sitter universe, and the role of string theory in the cosmological constant problem. The cosmological constant is a key component in the Friedmann equations, which describe the expansion of the universe. Observational evidence for a non-zero cosmological constant includes supernova results, the age of the universe, gravitational lensing, and CMBR anisotropies. Theoretical models such as quintessence and tachyonic scalar fields are discussed, along with the cosmic degeneracies they introduce. The review also addresses the problem of why the cosmological constant is so small compared to other energy scales in the universe, a challenge known as the cosmological constant problem. This problem is further complicated by the fact that the observed value of the cosmological constant is extremely fine-tuned, leading to the question of why it is positive and of such a small magnitude. The review highlights the importance of understanding the cosmological constant in the context of dark energy and its role in the accelerated expansion of the universe. It also discusses the implications of the cosmological constant for the structure formation in the universe and the constraints on dark energy from CMBR anisotropies. The review concludes with a discussion of the role of string theory in addressing the cosmological constant problem, emphasizing the need for a deeper understanding of the fundamental principles that govern the universe.