This chapter explores the concepts of genome size and the C-value paradox in plants, tracing the historical evolution of genetic understanding from Mendel’s foundational work on heredity to the elucidation of chromosomes by Boveri and Sutton. The chapter defines the genome as the complete set of genetic information in an organism and highlights the structural differences between prokaryotic and eukaryotic genomes. It introduces the term “C-value,” which quantifies DNA content and emphasizes its constancy across a species’ cells. Advancements in knowledge and the availability of high-throughput instruments over the past few decades have significantly enhanced our ability to measure the actual C-value of DNA within cells. Advances in measurement techniques over recent decades have led to significant insights into plant genome sizes, which can vary remarkably—up to 3000-fold—across the plant kingdom. Diversity in eukaryotic genome size also revealed a range from 2.3 Mbp in the parasitic microsporidian Encephalitozoon intestinalis to approximately 150,000 Mbp in the plant Paris japonica. The C-value has emerged as a vital parameter in research on biodiversity, taxonomy, and phylogeny. The chapter also addresses the complexities surrounding C-value consistency, particularly in cases such as maize, where supernumerary chromosomes and heterochromatin raise questions about the relationship between DNA content and chromosome number. Simultaneously, DNA size estimation in prokaryotes revealed substantial variation, ranging from 140 kilobase pairs (Kbp) to nearly 15 megabase pairs (Mbp). Eukaryotic genome sizes showed even greater diversity, with a range more than 60,000-fold, from 2.3 Mbp in the parasitic microsporidian Encephalitozoon intestinalis to approximately 150,000 Mbp in the plant Paris japonica. Ultimately, the chapter underscores how variations in genome size and structure reflect evolutionary processes and continue to intrigue scientific inquiry, paving the way for ongoing exploration in plant genomics.