Active particles are macroscopic, microscopic and nanoscopic in size and have propulsion speeds (typically) up to a fraction of a centimeter per second. The letters correspond to the artificial particles in Table 1 [16-39]. The insets show examples of biological and artificial swimmers. Three representative particles of macroscopic artificial active matter are displayed in the green box: robots, Hexbugs, and surfers (Adapted with permission from Ref. [2]. Copyrightc2016, American Physical Society).