Briefly summarizing, early studies on the role of Ca2+ in plants focused on the cell wall and on membrane permeability. At that time, there was no widespread appreciation that the [Ca2+]i might be very low and that this ion might be acting as a regulator of cytoplasmic processes. Botanists exploring Ca2+ effects in the concentration range between 0.1 and 100 mM were unlikely to see changes at the submicromolar level. The concept of Ca2+ as a regulator initially derives from studies of animal cells and only later in studies of plant cells. To see how this concept arose, I will focus briefly on Ca2+ in animal cell physiology, giving attention to the process of muscle contraction.