This review discusses the cosmological constant from both cosmological and field theoretical perspectives, focusing on conceptual and fundamental issues. It begins with an introduction to key issues related to the cosmological constant and a historical overview. The kinematics and dynamics of the standard Friedmann model of the universe are then summarized, followed by a detailed discussion of observational evidence for a non-zero cosmological constant, including supernova results, constraints from the age of the universe, gravitational lensing, and CMBR anisotropies. The review also explores theoretical models such as quintessence and tachyonic scalar fields. The latter part of the review delves into more conceptual and fundamental aspects of the cosmological constant, including alternative interpretations, relaxation mechanisms, the geometrical structure of de Sitter spacetime, thermodynamics, and the role of string theory. The review highlights the challenges and puzzles surrounding the cosmological constant, such as its fine-tuning and the need for a consistent theory of quantum gravity to understand its origin and behavior.This review discusses the cosmological constant from both cosmological and field theoretical perspectives, focusing on conceptual and fundamental issues. It begins with an introduction to key issues related to the cosmological constant and a historical overview. The kinematics and dynamics of the standard Friedmann model of the universe are then summarized, followed by a detailed discussion of observational evidence for a non-zero cosmological constant, including supernova results, constraints from the age of the universe, gravitational lensing, and CMBR anisotropies. The review also explores theoretical models such as quintessence and tachyonic scalar fields. The latter part of the review delves into more conceptual and fundamental aspects of the cosmological constant, including alternative interpretations, relaxation mechanisms, the geometrical structure of de Sitter spacetime, thermodynamics, and the role of string theory. The review highlights the challenges and puzzles surrounding the cosmological constant, such as its fine-tuning and the need for a consistent theory of quantum gravity to understand its origin and behavior.