This chapter aims to recapitulate the regulation of gene expression. Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as transfer RNA (tRNA) or small nuclear RNA (snRNA) genes, the product is a functional RNA. This chapter also discusses regulation of gene expression at different levels, lac operon, epigenetics, miRNA and SiRNA, mitochondrial DNA, mutation, patterns of inheritance and DNA polymorphisms. Gene amplification is the process by which the number of genes available for transcription is increased. Mutation is any permanent change in the primary nucleotide sequence regardless of its functional consequences. At the end of the chapter, multiple choice questions are also given along with their explanatory answers.