In the ongoing effort to build micro- and nanoscale machines, one of the key approaches is the bio-hybrid approach, which focuses on the use of biological constructs and engineered cells. As a natural extension of this concept to nanoscale communication, molecular communication is an umbrella term encompassing various communication systems that are built based on biological intra-and intercellular communication methods, most of which use molecules and molecular concentration as the information carrier. Compared to other proposed molecular communication systems such as diffusion-based communication and microtubular networks, calcium signaling is expected to provide a faster and more controllable system that is suitable for information dissemination and group behavior in nanoscale sensor networks. In this article, we give a general overview of calcium signaling, a novel communication paradigm that uses intercellular calcium waves in biology as a baseline, explain its capabilities, limitations, and some possible deployment scenarios. We also describe various open issues of this novel communication system and elaborate on some research directions for calcium signaling.