Calcium (Ca) and magnesium (Mg) are the most abundant divalent cations in plants. As a nutrient and a signaling ion, Ca levels in the cell are tightly controlled by an array of channels and carriers that provide mechanistic basis for Ca homeostasis and the generation of Ca signals. Although a family of CorA-type Mg transporters plays a key role in controlling Mg homeostasis in plants, more components are yet to be identified. Ca and Mg appear to have antagonistic interactions in plant cells, and therefore plants depend on a homeostatic balance between Ca and Mg for optimal growth and development. Maintenance of such a balance in response to changing nutrient status in the soil emerges as a critical feature of plant mineral nutrition. Studies have uncovered signaling mechanisms that perceive nutrient status as a signal and regulate transport activities as adaptive responses. This 'nutrient sensing' network is exemplified by the Ca-dependent CBL (calcineurin B-like)-CIPK (CBL-interacting protein kinase) pathway that serves as a major link between environmental nutrient status and transport activities. In this review, we analyze the recent literature on Ca and Mg transport systems and their regulation and provide our perspectives on future research.