Defining the genetic basis of disease requires the analysis of DNA and sometimes chromosomes. The vast majority of our genetic material is organized as immensely long DNA molecules, and changing just a single nucleotide out of the more than 3000 million nucleotides in our genome may cause disease. Sophisticated technologies have been developed to purify and manipulate genes, enabling studies on how they work and how they can be used therapeutically, and providing ways of analyzing mutations and studying the molecular basis of disease. We will outline how these approaches are used in later chapters. We describe here just the core technologies for purifying and analyzing DNA sequences.