Taking stock of circadian clock complexity

Circadian clocks are ubiquitous! This terse sentence has been used frequently in the last few years to mean not only that almost all higher organisms (and some bacteria) exhibit circadian rhythmicity controlled by an endogenous circadian clock,1 but also that many autonomous oscillators are present in different tissues or organs.2,3 Clock research also represents one of the more productive and faster evolving areas of biology, as evidenced by the extraordinary number of high profile publications that keep coming, thick and fast. This in turn has produced a market for reviews of molecular chronobiology, yet the ‘half-life’ of these reviews is significantly affected by this explosive accumulation of data and discourages potential authors to undertake this (thankless) task. I am therefore particularly pleased that a number of colleagues have committed themselves to write these contributions for this issue, which cover the most relevant aspects of circadian rhythmicity in different model systems.