Quantum metrology with nonclassical states of atomic ensembles explores how quantum entanglement can enhance the precision of measurements, particularly in atomic clocks and interferometers. This review discusses the theoretical and experimental foundations of quantum-enhanced metrology using atomic ensembles, focusing on generating and utilizing entangled states to surpass the standard quantum limit (SQL) in phase estimation. Key concepts include spin squeezing, quantum Fisher information, and the Heisenberg limit, which represent the ultimate sensitivity achievable with entangled states. The review covers various systems, such as trapped ions, Bose-Einstein condensates, and cold thermal atoms, and discusses techniques for generating entangled states through interactions like atom-atom collisions, atom-light interactions, and collective spin dynamics. Experimental results demonstrate phase sensitivity improvements beyond the SQL, with some systems achieving the Heisenberg limit. The review also addresses challenges such as noise, decoherence, and the need for robust entanglement generation and maintenance. Overall, the field highlights the potential of quantum technologies to revolutionize precision measurements in physics and engineering.Quantum metrology with nonclassical states of atomic ensembles explores how quantum entanglement can enhance the precision of measurements, particularly in atomic clocks and interferometers. This review discusses the theoretical and experimental foundations of quantum-enhanced metrology using atomic ensembles, focusing on generating and utilizing entangled states to surpass the standard quantum limit (SQL) in phase estimation. Key concepts include spin squeezing, quantum Fisher information, and the Heisenberg limit, which represent the ultimate sensitivity achievable with entangled states. The review covers various systems, such as trapped ions, Bose-Einstein condensates, and cold thermal atoms, and discusses techniques for generating entangled states through interactions like atom-atom collisions, atom-light interactions, and collective spin dynamics. Experimental results demonstrate phase sensitivity improvements beyond the SQL, with some systems achieving the Heisenberg limit. The review also addresses challenges such as noise, decoherence, and the need for robust entanglement generation and maintenance. Overall, the field highlights the potential of quantum technologies to revolutionize precision measurements in physics and engineering.