Dynamical systems is an important and thriving area that focuses on qualitative properties of time-dependent problems. Oscillatory behavior is easy to spot in a physical system and is a natural candidate for investigation. “Mixed-Mode Oscillations with Multiple Time Scales,” by Desroches, Guckenheimer, Krauskopf, Kuehn, Osinga, and Wechselberger, reviews a very specific class of oscillations that are easy to describe and illustrate (see Figure 1 on p. 212). But quantifying and analyzing this type of behavior is an ongoing challenge that has led to the development of new concepts and methodologies. “Mixed mode” refers to the co-presence of small- and large-amplitude oscillations along a trajectory. This article focuses on fast-slow ordinary differential equations (ODEs) where small-amplitude oscillations arise from local phenomena around a special point of the limiting system. Some intricate concepts are introduced through concrete examples, including the well-known Van der Pol equation. It is natural to think geometrically in this field, and the authors have produced some highly creative and, in many cases, stunning graphical illustrations to help the reader along. We are shown how mathematical advances can lead directly to important new insights about complex behavior. Computational experiments are a key investigative tool, and a customized approach, exploiting analytical insights, is often necessary. Indeed, as the treatment moves back and forth between theory and algorithmics, it becomes clear that these two aspects develop best in tandem. The comprehensive review in sections 2 and 3 is followed by four illustrative case studies. Tables 4–6 then scope the literature on mixed-mode oscillations in three categories: The authors also point to current hot topics, such as dealing with higher dimensional, spatially dependent or stochastic systems and calibrating models to real data. This work will be of interest both to analytically oriented readers looking to get a feel for the key concepts and open problems, and to those with particular applications in mind who wish to learn about the latest tools.