The most general definition of transcriptome assumes that it is a complete set of RNA molecules (transcripts) that are formed in a given organism. In response to exogenous and endogenous stimuli, transcriptome undergoes constant qualitative and quantitative changes. Monitoring these changes can provide information on how the living organisms function and deal with the changing environmental conditions. Recently, a new approach, called RNA sequencing or RNA-Seq has been applied to transcriptome-oriented studies. It is based on the next-generation sequencing and allows transcript discovery and quantification. This new approach permits identification of novel transcripts and products of their processing or partial degradation. Since RNA-Seq does not require information about the sequence of the reference genome, it can be used for transcriptome analysis in any organism. This feature combined with the high sensitivity and broad dynamic range of RNA-Seq experiments, make them much more versatile for high-throughput studies as compared to DNA microarray technology.