Behavioral patterns of inter-crab communications as shown in agonistic and sexual interactions are investigated in 26 genera within 9 aquatic and semi-terrestrial families of brachyurans. The investigation includes a review of literature and the author's observations. Species of the aquatic families, Dromiidae, Leucosiidae, Majidae, Parthenopidae, Cancridae, Portunidae, and Xanthidae, display agonistic activities mostly with more or less extended chelipeds. An encounter leads often to physical interactions, such as striking blows or grasping with chelae. Many of the semi-terrestrial ocypodids and grapsids, however, display with flexed chelipeds, showing the opponent the outer side of the chelae in a shield-like manner. Encounters usually follow a formalized set of patterns and only occasionally do they lead into a “wild fight”. Concerning sexual activities in many aquatic forms, a male catches and holds or cages a female between its legs for several days during the mating period. In many semi-terrestrial grapsids and ocypodids, patterns of up-down display of chelipeds have been observed during agonistic encounters, as well as during initial stages of mating. In other forms a special display of chelipeds occurs in courtship. In agonistic and sexual activities of aquatic forms intern-crab communication seems to depend mainly on tactile and possibly chemical stimuli. In the semi-terrestrial forms that have been investigated, mechanisms of communication are based predominantly on optical information.