This chapter discusses how genes and genomes change over time. It examines the molecular mechanisms that generate genetic diversity, and we consider how the information in genomes can be deciphered to yield a historical record of the evolutionary processes that have shaped these DNA sequences. The chapter looks at mobile genetic elements and consider how these elements, along with modern-day viruses, can carry genetic information from place to place and from organism to organism. Evolution is more a tinkerer than an inventor: it uses as its raw materials the DNA sequences that each organism inherits from its ancestors. DNA sequences that can move from one chromosomal location to another—are an important source of genomic change and have profoundly affected the structure of modern genomes. By comparing the DNA and protein sequences of contemporary organisms, we are beginning to reconstruct how genomes have evolved in the billions of years that have elapsed since the appearance of the first cells.