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Plants vary enonnously in their genome size, structure, organization and the chromosome number and shape as a consequence of millions years of evolution. Comparative cytogenetic analysis of different karyotypes indicated significant contribution ofchromosomal rearrangements to plant evolution. Chromosomal rearrangements such as inversion, translocation and duplication are common. They range from the part of a gene to chromosomal fragment and very often they are difficull for detection with traditional cytogenetic methods. Recent development of molecular cytogenetic techniques opened new possibilities for ana lysis of chromosome structure. Locali­zation of various DNA sequences, especially repetitive sequences, and recently BAC clones, enabled identification of particular chromosomes or whole genomes in nucleus of many plant species. These investigations have provided new data on chromosomal rearrangements and on great importance ofpolyploidization and diploidization processes in plant evolution. Molecular and cytological technologies have revealed many novel paleopolyploids, which have been traditionally considered as diploids. The most of angiospenns have experienced polyploidiza­tion in their evolutionary bistory. On the other hand, genetic and epigenetic modifications are leading factors promoting genetic diploidization. Five processes: polyploidization, transposon amplification, chromosome breakage, unequal homologous recombination, and illegitimate recombination are considered as the major mechanisms generating chromosomal variation during evolution. Polish cytogenetics has significant contribution to plant genome investigation. Karyotyping and genome size analyses have been developed in many laboratories. Chromosome engineering techniques were introduced to basie studies and plant breeding programs. Nowadays the newest techniques of molecular cytogenetics are widely applied to phylogenetic investigations.