Calcium is an essential ion for life due to its involvement in cellular signaling. The ability of magnetic resonance imaging (MRI) to follow its fluctuations at the molecular and cellular level and on a large scale makes this technique very helpful for the visualization and understanding of critical biological processes. This can successfully be 24achieved through the development of effective MRI contrast agents that are sensitive to Ca(II). A great number of probes have been investigated in the past two decades that use different MRI methodologies based on relaxation processes, chemical exchange saturation transfer, and use of nuclei other than 1H or hyperpolarized MRI. The progress has been spectacular, resulting in a few very successful sensors that have allowed the first in vivo monitoring of intra- and extracellular Ca(II) concentration changes. This chapter summarizes the current state-of-the-art applications in this field and provides some practical insights that may be relevant for the future advancement of this exciting field and the development of functional molecular imaging.