Transcription of a gene produces an mRNA. In prokaryotes, little or no processing is needed before the transcribed mRNA is taken up for translation. In eukaryotes, extensive processing is performed between the mRNA synthesis and translation. These modifications are crucial for diversifying the proteome and regulating gene expression. In most eukaryotes, some of the steps occur co-transcriptionally in the nucleus. These processing steps are vital for mRNA quality control and regulation of gene expression. We describe several significant nuclear processing steps. These steps are splicing, capping, polyadenylation, chemical modifications, RNA editing, and nuclear export. We describe the mechanisms and, in some cases, structural details of the process. In the splicing section, we also discuss alternative splicing, a significant source of proteome diversity. Similarly, when we discuss polyadenylation, we also discuss alternative polyadenylation. Although mRNAs may carry many different modifications, we focus on two most abundant modifications: N6-methylation of adenosine and conversion of uridine to pseudouridine. We discuss mRNA editing, another source of proteome diversity. Finally, we discuss nuclear mRNA export, a key point at which gene expression is often regulated. In a later chapter, we will discuss the regulatory mechanisms at many of these steps.