Asymmetric cleavage plays an important role in Caenorhabditis elegans embryogenesis. In addition to generating cellular diversity, several early asymmetric cleavages contribute to the spatial organization of the embryo. Genetic and molecular analyses of several genes, including six par genes and the mex-1 and mes-1 genes, together with experimental embryological studies, have provided insights into mechanisms controlling polarity and spindle orientations during these cleavages. In particular, these studies focus attention on microfilament-based motility and changing protein distributions at the cell cortex.