New information and discussions regarding Mesozoic diatoms presented over the last decade advanced our knowledge of their origin and early history. The oldest confirmed centric diatom fossils are presented here from the earliest Cretaceous, and araphid and raphid pennate diatoms now date from the Late Cretaceous; all from terrestrial sediments. Molecular sequencing helped clarify relationships between diatom lineages, and verify the position of diatoms within the heterokontophytes. Molecular clock approaches estimate a diatom origin near ~135 Ma, but not before 240 Ma. Biomarkers in marine sediments are able to trace a diatom presence back to the mid-Cretaceous, even when siliceous fossils are absent. Seasonal growth and encystment cycles in Late Cretaceous planktonic marine diatoms are now well documented. A biostratigraphic framework for the Late Cretaceous Arctic will aid regional and global biostratigraphic correlations. The systematic position of many new taxonomic groups is now included within a more natural classification scheme that better reflects phylogenetic relationships evident in molecular data and affirmed by biostratigraphic micropaleontology. Discussions regarding the impact of diatoms on several global systems are maturing, as more information becomes available. Four stages in diatom evolution are proposed to explain the history of radiation, extinction, and expansion into new environments and habitats during the Mesozoic.