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Summary There are four methods of analysing the vertical migration of plankton in the field: (a) One day and one night observation at the surface. (b) One day and one night observation at depths between surface and the bottom. (c) Observations at long time intervals at all depths during day and night. (d) Observations at short time intervals at all depths during day and night. The last method, using horizontal closing tow‐nets with flow meters at measured depths, shows that migration consists of four parts: (a) ascent from the day depth; (b) midnight sinking; (c) dawn rise; (d) descent to the day depth. This pattern has been shown to be modified by wind (or state of sea), rain, sun on a calm sea, extremes of temperature, age of the individual animal and possibly by phytoplankton concentrations. These different parts of migration have been examined in detail and linked together in a single and continuous process. Ascent in the evening and descent in the morning become the continuous change of day depth with the changing penetration of light; the midnight sinking is assumed to be due to a passive state in full darkness; the dawn rise is a return by the animals to the mean optimum light intensity for the population. This is supported by the fact that the order in time of arrival at the surface for some fresh‐water species is the same as the order in depth of these species in full daylight. Theoretical interpretations of migration are reviewed. It is argued that the most likely theory has as its basic assumption the idea of an optimum light intensity. Experiments on migration are divided into three parts: (a) those showing that randomly oriented movement can take place; (b) those showing that the speeds of individual animals can vary with varying light intensities; (c) those showing that animals in full darkness will move at minimal speeds or not at all. It is concluded that the vertical migration of planktonic Crustacea is mediated by the change in light penetration throughout the day. Animals aggregate in an optimum band of light intensity. They have the capacity of moving phototactically, i.e. moving at constant speed towards or away from light; they also have the capacity of moving photokinetically, i.e. moving at speeds that vary with varying light intensity. In full darkness they move probably at minimal speeds. Migration between periods of full darkness may be operated by phototaxis, photokinesis or by a combination of these mechanisms.