Transcriptome analysis or global gene expression profiling is a powerful tool for discovery as well as -understanding biological mechanisms in health and disease. We present in this chapter a description of methods used to isolate mRNA from cells and tissues that has been optimized for preservation of RNA quality using clinical materials and implemented successfully in several large, multicenter studies by the authors. In addition, two methods, gene expression microarrays and RNAseq, are described for mRNA profiling of cells and tissues from clinical or laboratory sources.