Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase. Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language. Steps of transcription, post-transcriptional modification, and different classes of RNA are discussed in this chapter. This chapter also covers differences between replication and transcription, similarities between transcription and replication, applications of alternative mRNA processing, different classes of RNA, messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), arms of tRNA, small nuclear RNA (SnRNA), micro RNA (miRNA), silencing RNA or small interfering (SiRNA), long noncoding RNA (lnc RNA), and hnRNA (heteronuclear RNA).