Introduction to focus issue: intracellular Ca2+ dynamics--a change of modeling paradigm?

Intracellular Ca(2+) concentration dynamics have been perceived as a prototypical deterministic intracellular reaction-diffusion system in biophysics for many years. Recent experimental findings challenge that view and suggest them to be fluctuation driven. That renders this system interesting for nonlinear physics, in general, since we can study the emergence of macroscopic behavior from mesoscopic dynamics. In particular, we can observe the random elemental events, called puffs, and the macroscopic pattern with the same experimental means. Here, we give a short introduction to the current discussion on theoretical and modeling concepts, and this Focus Issue reflecting it.