General inspection of a role performed in the cell by RNAs allows us to distinguish three major groups of transcripts: I . protein-coding mRNAs, II . non-coding housekeeping and III . regulatory RNAs. The housekeeping RNAs include RNA classes that are generally, constitutively expressed and whose presence is required for normal function and viability of the cells. On the other hand, a group of regulatory RNAs includes RNA species that are expressed at certain stages of organism development or cell differentiation or as a response to external stimuli and can affect expression of other genes on the levels of transcription or translation. Non-coding RNA transcripts form a heterogeneous class of RNAs that can not be characterized by a single specific function. Initially, the term non-coding RNA (ncRNA) was used primarily to describe polyadenylated and a capped eukaryotic RNAs transcribed by RNA polymerase II, but lacking long open reading frames. Now, this definition can be extended to cover all RNA transcripts that do not show protein-coding capacity and is sometimes used to describe any RNA that does not encode protein, including introns. This book is an in-depth look at the function of Non-Coding RNAs and their relationship to Molecular Biology and Molecular Biology.