Taxonomy is defined as the science, laws or principles of classification within the plant, animal and Protista kingdoms. Plant taxonomy is involved with identification, naming, classification and the theories of classification based upon evolutionary and genetic relationships. The earlier plant classification systems were based strictly on morphological attributes (i.e. structure and form). Current phylogeneyic systems of classification are based more on genetic and evolutionary relationships. In recent years there has been considerable change and differences of opinion as more advances are made in understanding genetic relationships and codes for each type of organism.

Plant classification is a complex and controversial matter. In general, botanist fall into two categories. The first category studies small groups of plants and tends to emphasize differences (splitting), while the other group works on a larger scale and tends to emphasize resemblances (lumping). Thus what one botanist regards as a distinct family may be regarded by another as a subfamily, etc.

During the past century there have been at least a dozen different plant classification systems or major modifications to existing systems proposed. The following table presents four different, but widely accepted, classification systems. This table compares these four systems for all the broader categories in the plant kingdom above family for the redwood family, Taxodiaceae. Subdivisions within the redwood family are discussed on the Redwood Family page.

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