Milk calcium (Ca) content has long been considered to be determined by milk protein content, with a strong influence of genetics of cows. Milk mid-infrared spectrometry analyses allowed showing a high heritability of milk Ca contents and a genetic correlation between this parameter and protein contents. This review of INRAE results aimed to illustrate that environmental factors also induce variations in milk Ca content. A survey based on the collection of 200,000 individual milk samples in the major areas of milk production in France showed that calendar month and cow feeding strategy affected milk Ca content, which dropped in the spring during grazing turnout and was lower when cows were fed fresh and conserved grass rather than corn silage. Complementary experimental studies clearly showed that the stage of lactation, which is an identified cause of milk Ca content variation, only explained a limited part of this variability. Thanks to a meta-analysis, based on experiments comparing diets in cows with similar stage of lactation, we showed a specific effect of diets on milk Ca content. Total, soluble and colloidal calcium contents of milk and ratio between milk Ca and protein contents were lower when diets were based on pasture rather than on corn silage, with intermediate values with diets based on grass silage or hay. Lower milk Ca contents with grass-based diet were not explained by an increased dietary cation anion difference. For some other environmental factors influencing milk Ca content, such as digestible protein and net energy balance or day length, we could observe parallel variations of milk Ca and casein contents. Milk Ca variations may be related to variations of the expression of mammary genes involved in calcium secretion pathways. Even though we observed a huge increase in knowledge in the regulation of Ca secretion in milk these last 15 years, the underlying mechanism for environmental milk Ca variation remain to be elucidated.