The genomes of five species belonging to the grass family have been fully sequenced and provide insight into the structural organization and evolution of grass genomes. Comparative analyses have shown that genes and repeats are organized differently in small genomes compared to large genomes. Small genomes show a clear partitioning between gene-rich euchromatic and gene-poor pericentromeric regions. This is far less the case in larger genomes because many repeats are also interspersed between single genes or small gene islands. This organizational pattern may enhance erosion of colinearity because of the inherent mutagenic effects of transposable elements. Factors contributing to genome diversification, which is not constant in either space or time, are rapid turnover of repeats, chromosomal rearrangements, gene loss or differentiation following gene duplication and potentially the creation of new genes from transposable element-acquired gene fragments.