The present perspective summarizes information on the variability of krill population parameters, which can assist in interpreting fluctuations in euphausiid populations. This requires knowledge about potential longevity of an individual as well as the age composition of populations. Further key parameters of the life cycle are discussed for a number of euphausiid species from oceans around the world. In a variable environment, species may show variations in sex ratio, breeding season, larval development, as well as functional characteristics such as natural mortality and productivity. Conceptual models based on results for some krill species indicate that environmental disturbances create instability in populations by reducing the recruitment success, which finally can lead to a decreasing population size.