Studying middle atmospheric trends requires knowledge on all relevant sources of variability affecting atmospheric parameters of interest. The Earth’s middle atmosphere is subject to variability driven by a variety of different processes from above and below. Quasi 27-day signatures, presumably driven by the solar 27-day rotational cycle, have been identified in several middle atmospheric parameters. The identification of these signatures is usually straightforward. However, the underlying physical and/or chemical mechanisms are typically poorly understood and involve both photochemical and dynamical effects. For some atmospheric parameters the sensitivity factors (% change divided by % change in solar forcing) for the 27-day and the 11-year cycle agree within uncertainties, which may allow constraining the 11-year atmospheric response by studying the 27-day response. This contribution will provide an overview of the current scientific understanding of 27-day signatures in the Earth’s middle atmosphere. In addition possible mechanisms and open questions will be discussed.