Adult cardiomyocytes exhibit complex Ca2+ homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca2+ sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca2+ homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as neonatal cell culture and stem cells.