We review here the progress that has been achieved using molecular cytogenetics to analyze the genome structure of sugarcane (Saccharum spp) and banana (Musa spp), two crops that are polyploid, of interspecific origin and with chromosomes not distinguishable by their gross morphology. In Saccharum, molecular cytogenetics enabled us to determine the basic chromosome number of two species, Saccharum officinarum and S. spontaneum, involved in the origin of modern cultivars, to quantify the proportion of chromosomes of these species in the genome of modem cultivars, to assess the extent of interspecific chromosome recombination and to clarify the origin of the related species S. barberi. These techniques are also used to monitor introgression with related genera. In Musa, GISH enabled us to differentiate the four genomes involved in banana cultivars and allowed us to determine the genome constitution of several cultivars. FISH was used to analyze the distribution of repeated sequences along the genome.