Light is the most important Zeitgeber to entrain the circadian clock of the fruit fly Drosophila melanogaster to the 24-h cycle on earth. The fruit fly's circadian clock is very light sensitive, mainly because about half of the 150 clock neurons in the fly's brain express the blue-light photopigment, Cryptochrome, which provokes an immediate degradation of the clock protein Timeless upon activation by light. Consequently, Drosophila's molecular clock can reset very fast to measure the changes in environmental-lighting conditions. However, usually the responses of the molecular clock to light are not directly measured, but conclusions about entrainment of the circadian clock are drawn from recording the flies' locomotor activity rhythms. Here, we review how the flies' locomotor activity can be recorded under different light regimes and how entrainment can be analyzed and properly judged. We also summarize the influence of different recording and lighting methods on the flies' activity pattern, highlight their advantages and disadvantages, and stress general pitfalls.