A clearer understanding of carcinogenesis is emerging with our rapidly expanding knowledge of genetics. At the same time there remain issues surrounding genetics and genetic testing, which are very important. Cancer results from a breakdown in the genetic control of cell growth and behaviour. The study of genetic changes associated with different types of cancer has been under- way for over 40 years and has become central to the diagnosis and management of many cancers. For example, most leukaemias are associated with specific chromosomal rearrangements that activate the genetic messages which stimulate growth of that cell type. One of the earliest discoveries, the Philadelphia chromosome in chronic myeloid leukaemia, was later shown to involve a translocation joining together pieces of chromosomes 9 and 22. This produced an abnormal gene capable of generating a tyrosine kinase like product. Recently, a highly effective drug designed to block that gene product, imatinib, has been approved for clinical use. It is now essential for the effective management of most leukaemias to have access to high quality cytogenetic diagnosis. These techniques are being extended into the use of molecular diagnostic techniques. A good example is detection of the characteristic amplification of the proto-oncogene Nmyc in neuroblastoma and her2 in breast cancer. These changes are somatic errors, mistakes which arise in a cell in the body at some time after conception. In almost all cases, a series of genetic errors must occur before a cell becomes capable of uncontrolled growth and spread to other sites. In some individuals, a genetic error in the germline that predisposes them to cancer affects every cell in the body. Such changes can be inherited, resulting in families with multiple affected members. The last decade has seen an upsurge in discoveries of the genes that underlie these hereditary forms of cancer. The attraction of this research has been that it provides a means of more accurate diagnosis, and in some cases allows presymptomatic diagnosis. Any gene which, when defective, predisposes to malignancy is usually a key part of an important pathway. As a result, discovery of these genes has led to a better understanding of the causes of common cancers. The classic example is the APC gene on chromosome 5 which underlies the rare dominant syndrome FAP. In most colorectal adenocarcinomas both copies of this gene are inactive, a change which is apparent in early adenomas. The identification of a pathological mutation in the APC gene, typically a frameshift mutation distal to the catenin binding site in exon 15, is of great clinical value as it allows accurate identification of other family members who will need regular endoscopy and prophylactic surgery. Of equal importance is the ability to discharge with confidence those family members who have not inherited the defective copy of the gene. A range of similar cancer syndromes are now amenable to molecular diagnosis; multiple endocrine neoplasia, Von Hippel Lindau syndrome, juvenile polyposis and neurofibromatosis type 2 are important examples of dominant syndromes. Recessive syndromes include Fanconi’s anaemia and Bloom’s syndrome, both of which are examples of defective DNA repair. Provision of diagnostic services for such disorders needs to be organised at regional, national and sometimes supranational levels to ensure an appropriate level of quality assurance and technical expertise.