The transformation of a normal into a malignant cell is a multistep mechanism, which involves various alterations on the molecular and genetic level. These molecular alterations occur spontaneously or are induced by carcinogens (e.g. naphthylamine--a component of cigarette smoke and one of the most important carcinogens leading to bladder tumor carcinogenesis). This report summarizes some of the most important molecular and genetic alterations in bladder cancer. As in most other malignancies the generation of bladder cancer is caused by the accumulation of various molecular changes. The expression of oncogenes (ras, erbB-2 and EGF receptor), tumor-suppressor genes (Rb, p53), cell-cycle genes (p15, p16) and DNA-repair genes is altered mostly by mutation or chromosomal aberration. Loss of heterozygosity of chromosome 9p and 9q has been shown to be a crucial event in the transition of normal urothelium to papillary transitional cell carcinoma while p53 is primarily involved in the development of carcinoma in situ.