This article reviews the theory and experiments of quantum metrology using nonclassical states of atomic ensembles. It highlights the potential of entanglement to enhance the sensitivity of precision measurements, such as phase estimation in atomic clocks and atom interferometers. The review covers the fundamentals of collective spin systems, phase estimation techniques, and the role of entanglement in improving metrological performance. It also discusses the generation of entangled states in Bose-Einstein condensates and trapped ion systems, and the impact of noise on interferometric protocols. The article emphasizes the importance of quantum Fisher information and the quantum Cramér-Rao bound in characterizing the optimal sensitivity of interferometers. Finally, it explores the practical challenges and future directions in quantum metrology with atomic ensembles.This article reviews the theory and experiments of quantum metrology using nonclassical states of atomic ensembles. It highlights the potential of entanglement to enhance the sensitivity of precision measurements, such as phase estimation in atomic clocks and atom interferometers. The review covers the fundamentals of collective spin systems, phase estimation techniques, and the role of entanglement in improving metrological performance. It also discusses the generation of entangled states in Bose-Einstein condensates and trapped ion systems, and the impact of noise on interferometric protocols. The article emphasizes the importance of quantum Fisher information and the quantum Cramér-Rao bound in characterizing the optimal sensitivity of interferometers. Finally, it explores the practical challenges and future directions in quantum metrology with atomic ensembles.