Polyploidy is a common mode of evolution in flowering plants. The profound effects of polyploidy on gene expression appear to be caused more by hybridity than by genome doubling. Epigenetic mechanisms underlying genome-wide changes in expression are as yet poorly understood; only methylation has received much study, and its importance varies among polyploids. Genetic diploidization begins with the earliest responses to genome merger and doubling; less is known about chromosomal diploidization. Polyploidy duplicates every gene in the genome, providing the raw material for divergence or partitioning of function in homoeologous copies. Preferential retention or loss of genes occurs in a wide range of taxa, suggesting that there is an underlying set of principles governing the fates of duplicated genes. Further studies are required for general patterns to be elucidated, involving different plant families, kinds of polyploidy, and polyploids of different ages.