Globular clusters are large and dense agglomerate of stars. At variance with smaller clusters of stars, they exhibit signs of some chemical evolution. At least for this reason, they are intermediate between open clusters and massive objects such as nuclear clusters or compact galaxies. While some facts are well established, the increasing amount of observational data are revealing a complexity that has so far defied the attempts to interpret the whole data set in a simple scenario. We review this topic focusing on the main observational features of clusters in the Milky Way and its satellites. We find that most of the observational facts related to the chemical evolution in globular clusters are described as being primarily a function of the initial mass of the clusters, tuned by further dependence on the metallicity—that mainly affects specific aspects of the nucleosynthesis processes involved—and on the environment, that likely determines the possibility of independent chemical evolution of the fragments or satellites, where the clusters form. We review the impact of multiple populations on different regions of the colour–magnitude diagram and underline the constraints related to the observed abundances of lithium, to the cluster dynamics, and to the frequency of binaries in stars of different chemical composition. We then re-consider the issues related to the mass budget and the relation between globular cluster and field stars. Any successful model of globular cluster formation should explain these facts.

球状星团是巨大而致密的恒星团聚体。与较小的星团不同的是，它们表现出一些化学演化的迹象。至少出于这个原因，它们介于疏散星团和大质量天体（例如核星团或致密星系）之间。虽然一些事实已得到充分证实，但越来越多的观测数据揭示了一种复杂性，迄今为止，无法在简单的场景中解释整个数据集。我们回顾这个主题，重点关注银河系及其卫星星团的主要观测特征。我们发现，与球状星团化学演化相关的大多数观测事实被描述为主要是星团初始质量的函数，并通过进一步依赖金属丰度进行调整——这主要影响所涉及的核合成过程的特定方面——在环境方面，这可能决定了碎片或卫星（簇形成的地方）独立化学演化的可能性。我们回顾了多个种群对颜色星等图不同区域的影响，并强调了与观测到的锂丰度、星团动力学以及不同化学成分恒星中双星频率相关的限制。然后我们重新考虑与质量预算以及球状星团和场星之间的关系有关的问题。任何成功的球状星团形成模型都应该解释这些事实。