Crustacea (or Pancrustacea) have explored virtually all possible milieus in different parts of their life cycle, including freshwater, marine, and terrestrial habitats, and even the air (pterygote insects). Many crustacean taxa display complex life cycles that involve prominent shifts in environment, lifestyle, or both. In this chapter, the overwhelming diversity of crustacean life cycles will be explored by focusing on changes in the life cycles, and on how different phases in a life cycle are adapted to their environment. Shifts in crustacean life cycles may be dramatic such as those seen in numerous decapods and barnacles where the development involves a change from a pelagic larval phase to an adult benthic phase. Also, taxa remaining in the same environment during development, such as holoplanktonic Copepoda, Euphausiacea, and Dendrobranchiata, undergo many profound changes in feeding and swimming strategies. Numerous taxa shift from an early larval naupliar (anterior limbs) feeding/ swimming system using only cephalic appendages to a juvenile/ adult system relying almost exclusively on more posterior appendages. The chapter focuses mainly on nondecapods and is structured around a number of developmental concepts such as anamorphosis, metamorphosis, and epimorphosis. It is argued that few crustacean taxa can be characterized as entirely anamorphic and none as entirely metamorphic. Many taxa show a combination of the two, even sometimes with two distinct metamorphoses (e.g., in barnacles), or being essentially anamorphic but with several distinct jumps in morphology during development (e.g., Euphausiacea and Dendrobranchiata). Within the Metazoa the Crustacea are practically unrivalled in diversity of lifestyles involving, in many taxa, significant changes in milieu (pelagic versus benthic, marine versus terrestrial) or in feeding mode. Probably such complex life cycles are among the key factors in the evolutionary success of Crustacea.