Polyploidy or whole-genome duplication (WGD) is a major evolutionary force in green plants. Genomic analyses reveal that it is much more widespread across Viridoplantae than previously thought. Unexpected examples of deep reticulation and WGD continue to be revealed as plant genomes are sequenced. In angiosperms, WGD is often (but not always) associated with bursts in species diversification, playing a major role in many clades in stimulating novelty. However, in gymnosperms, in contrast, most shifts in diversification are decoupled from WGD, revealing underlying differences between different major clades of green life. At a finer scale, recently formed (within the past 150 years), naturally occurring allopolyploid species have provided models for detailed study of the early events that follow genome doubling. Genetic and genomic studies reveal that polyploidy is a force of rapid change, with impacts that are seen immediately following genome duplication or in the earliest generations following WGD. These changes include not only long-recognized alterations in nuclear volume and cell size and their concomitant structural/functional impact, but also genomic changes including gene loss and silencing, as well as chromosome modifications including reciprocal aneuploidy and inter-genomic translocations. A recurring theme following WGD is the generation of novelty, whether it be via unique patterns of methylation, genome-wide changes in gene expression, or chromosome alterations. Many aspects of the early post-polyploidization process seem intrinsic; evolution repeats itself across independent polyploidizations in nature and in synthetic polyploid lines. These intrinsic changes seem to be driven simply by features of the parental genomes themselves.