The term "Gene" refers to a segment of DNA present inside the genome that code for a protein. Depending on the organism, a particular genome may encode thousands of diverse types of genes responsible for various functions. These genes are differentially expressed and tightly regulated as needed at different stages of cell and physiological functions including cell differentiation and development. Gene expression may be influenced by various environmental and stimuli such as temperature, nutrients, hormones, stress etc. Though the fundamental mechanism of transcription (production of mRNA from the protein coding genes) is very similar in different organisms, the mechanism of gene regulation and the machineries involved are distinct for different organisms. In the post genomic and epigenomic era, it is increasingly being recognised that gene expression is not only controlled gene sequences but also various other non-genomic factors that include protein and DNA modifications and non-coding RNAs. Various aspects of gene expression in prokaryotes and eukaryotes, the mechanism of transcription and gene regulation and influences of various environmental factors are highlighted in different chapters of this book.