In the wrap-up to Lecture 1, we used the vocabulary list that was assembled by Jürgen Aschoff. I can't help but admire him for taking on this task. It must have been somewhat controversial at the time to sit down and put definitions to some of the concepts, that were just in the early process of being established. Aschoff's initiative, as well as Pittendrigh's, in listing the formalisms of circadian clocks, have advanced our field enormously. Indeed, that chronobiologists have described the qualities of the circadian clock, that they've found many genes in the molecular clockwork was only possible due to extensive formal description of how clocks behave under different conditions. In his lecture about formalisms and entrainment, Till will use Pittendrigh's paper published in 1960, to guide you through these important formalisms. What circadian clocks do in constant conditions, and how systematically they respond to environmental signals called zeitgebers, which they use to synchronize their daily rhythms to the 24 hour day. You will need this knowledge to comprehend how clocks work, to understand the special aspects of circadian experiments, and to appreciate the differences between rhythms that are controlled by the endogenous circadian mechanism, and those that are simply reactions to changes in the environment.