Advances in DNA technology and the discovery of DNA polymorphisms have opened up the possibility of creating data bases of individuals' DNA for criminal investigation purposes. In many European countries laws are being drawn up or have been passed to regulate such data bases. In Spain theres is a need for an urgent ethical and legal debate on ther appropriateness. The article is divided into two parts. In the first, the authors discuss the technical principles underlying these data bases. In the second, they set out their legal and ethical views on regulation in Spain.