Minichromosomes have been extensively used as tools for revealing the functional structures of eukaryotic chromosomes. However, the definition of a minichromosome is still ambiguous. Based on previous reports on various eukaryotes, minichromosomes are defined here to be chromosomes that are smaller than one third the size of the smallest chromosome in the given species. In Arabidopsis thaliana, therefore, chromosomes <8.5 Mb in length are classified as minichromosomes, although to date only six different minichromosomes have been found or created, probably due to their extremely small sizes that limit detection. Minichromosomes vary from 1.7 to 8.4 Mb in length and are much shorter than authentic chromosomes (25.3 to 38.0 Mb). Linear and circular minichromosomes have been identified, and both types are maintained as experimental lines. Most of the circular, ring-shaped minichromosomes in Arabidopsis are relatively stable at mitosis and transmissible to the next generation, regardless of the centromere form (dicentric or monocentric). Recently, a ring minichromosome was artificially generated by a combination of the Cre/LoxP and Ac/Ds systems. This artificial ring chromosome, AtARC1, has several advantages over the previously reported minichromosomes as a chromosome vector; therefore, this method of generating artificial ring chromosomes is expected to be improved for application to other plant species including important crops.