Circadian clocks synchronize the physiology and behavior of most animals with the day to night cycle. A fundamental property of the molecular pacemakers generating circadian rhythms is their self-sustained nature: they keep oscillating even under constant conditions, with a period close to, but not exactly, 24 h. However, circadian pacemakers have to be sensitive to environmental cues to be beneficial. They need to be reset every day to keep a proper phase relationship with the day to night cycle, and they have to be able to adjust to seasonal changes in day length and temperature. Here, we review our current knowledge of the molecular and neural mechanisms contributing to the plasticity of Drosophila circadian rhythms, which are proving to be remarkably sophisticated and complex.