The DNA sequencing module describes how next-generation sequencing uses the chemistry of DNA replication to determine the nucleotide sequence of a DNA fragment. For RNA-seq, we use the same technology; however, because we are starting with RNA and not DNA, we must perform several chemical changes before the RNA molecules are ready to use with DNA sequencing technology. We call the collection of molecules that are ready for sequencing an “RNA-seq library.” Here, we will review how to prepare RNA molecules for sequencing, a process called “RNA-seq library preparation.”