Feulgen staining represents a staining method to quantitatively document the DNA content of a nucleus. Thus it is an excellent and straightforward method to reflect the irregular increase in DNA content of a malignant cell as a sign of genetic instability. Genetic instability of the tumour cell is the key feature of the 2004 WHO classification of bladder tumours, in which flat and papillary neoplasia are grouped into low- and high-grade lesions. "High grade" represents the tumor with genetic instability and consequently a higher likelihood of progression. Concomitant distinct genetic aberrations other than the numeric ones are increasingly identified as discriminators and help group the entities. The current status of genetic investigations, especially those in precancerous lesions, will be outlined in this review in the context of morphology (histology and cytology) as well as clinical situation.