Billions of animals are adapted to a travelling life, making regular return migrations between more or less distant living stations on Earth by swimming, flying, running or walking (Figure 1). Extremely long migrations are completed annually by whales between calving areas in warmer waters and feeding areas at higher latitudes in either hemisphere. The longest oceanic migrations among sea turtles and fish are often undertaken by younger immature individuals during a period of several years before they start their more regular return visits to breeding and spawning sites. Among adult leatherback turtles, intervals of several years between successive breeding events leave enough time for extremely long journeys. Famous among bird migrants are arctic terns, showing the longest known annual migration circuit of about 50,000 km. Bar-tailed godwits breed in Alaska and winter in New Zealand and make the longest known non-stop flapping flights, lasting more than two hundred hours and covering up to 12,000 km across the Pacific Ocean. Their total annual migration circuit extends over 30,000 km covered in three main flights (Figure 1). Although diapause with hibernation as egg, pupae, larvae or adult is an important strategy among insects, there are also examples of impressive migrations. Monarch butterflies complete an annual circuit up to 9,000 km in North America in four generations (for more detail, see the review by Steven Reppert in this issue), and the globe skimmer (a dragonfly) presumably exploits the monsoon rains in India and rainy seasons in southern and equatorial Africa in a 15,000 km circuit in four generations (Figure 1). In comparison with swimmers and flyers, animals that migrate by running or walking cover shorter distances. Caribous migrate between boreal forest and tundra over a total distance of not much more than 1000—2000 km per year. Zebras make the longest migrations in Africa, covering at least 500 km, which is just a little bit longer than the well-known wildebeest migration circuit in Serengeti. Alerstam and Bäckman introduce the ecological factors influencing the way animals migrate and how this branch of ecology has developed and grown.