Observations of chromosome behavior have suggested that it is sister chromatid cohesiveness which is primarily responsible for maintenance of chiasmate association between pachytene and anaphase of the first meiotic division and also for maintenance of sister centromere association until anaphase II. These associations seem essential for assurance of normal distribution of chromosomes into gametes (except in organisms in which alternative means have evolved, such as the male of Drosophila melanogaster). Sister chromatid cohesiveness is also found in varying degrees at mitosis. Reports of observations that are relevant to the nature of this cohesiveness are reviewed here with particular attention to behavior under a variety of conditions which include ploidy changes, presence of mutation effects, chromosome rearrangements, and experimental treatments. Attention is focused on constraints imposed upon model building by the observations, and also on directions for future study, which seem promising.