The amount of DNA in the nucleus of a cell is commonly referred to as the genome size or C-value and people have been estimating this character in plants and animals for over 50 years. Today, with data available for over 7,000 species (Table 19.1), land plants (embryophytes) are the best studied of the major taxonomic groups of eukaryotes. This chapter provides an overview of what is currently known about the diversity of genome sizes encountered in land plant groups and considers how such diversity might have evolved. The chapter by Greilhuber and Leitch (2012, this volume) will discuss the impact of this diversity on plants in terms of how differences in genome size have an impact at all levels of complexity, from the nucleus to the whole organism. This chapter should also be read with reference to those by Weiss-Schneeweiss and Schneeweiss (2012, this volume) who explore intra- and inter-specific chromosome complexity across angiosperms, including polyploidy and dysploidy, Murray (2012, this volume) who discusses gymnosperm chromosomes, and Barker (2012, this volume) who considers the chromosomes of monilophytes and lycophytes.