Microleakage is an important topic in dentistry.Numerous laboratory studies have found that microleakage is usually unavoidable whilst clinical studies report substantially less pessimism about the sealing ability of dental restorations. This work is presented on various forms of leakage in dental restorations and endodontics, distinguishing leakage, micro-leakage and nano-leakage. The phenomena, causes and methods to reduce leakage are described and the clinical relevance of in vitro and in vivo leakage studies evaluated and their usefulness and reliability.