The concentration of many proteins inside cells oscillates with time. These oscillations can be sustained or damped, and are often on timescales of minutes to hours. As the number of known examples of such oscillating systems increases, it is more and more relevant to try to understand whether there are common design principles in their regulatory circuit. I will discuss some of these examples and show that all of them are characterized by the presence of a negative feedback loop in the regulatory network. This result allows to extract informations about the underlying network structure from the time series of the concentrations.