Two different methods of presenting quantitative gene expression exist: absolute and relative quantification. Absolute quantification calculates the copy number of the gene usually by relating the PCR signal to a standard curve. Relative gene expression presents the data of the gene of interest relative to some calibrator or internal control gene. A widely used method to present relative gene expression is the comparative C_T method also referred to as the 2−ΔΔC_T method. This protocol provides an overview of the comparative C_T method for quantitative gene expression studies. Also presented here are various examples to present quantitative gene expression data using this method.