The key regular functions of microRNAs (miRNAs) were only discovered a few years ago, when it became evident that miRNAs play important roles in gene regulation. Today, more than 700 miRNAs have been identified from the human genome. Because of their regulatory functions in transcription, signal transduction, cell cycle regulation, proliferation, cell growth, apoptosis, cell metabolism and neurogenesis, malfunctioning miRNAs have been suggested to affect a number of diseases. Indeed, absence of miRNAs or their mutations has been associated with various types of cancers and other diseases such as neurodegenerative, autoimmune, cardiovascular, infectious, metabolic and oncologic diseases. This has been confirmed by genetic analysis of patient samples. The disease association has attracted potential diagnostic and therapeutic applications of miRNAs. A significant issue to achieve successful therapy is the delivery of miRNAs as problems of targeted and long-lasting presence of miRNA exist. Different approaches have included the modification of miRNAs and the use of viral delivery vectors.