This paper discusses the nature of the phenomenon of "mud volcanism" with respect to degassing of deeply buried sediments. Mud volcanoes are defined as geological structures and their main elements are described. Based on the nature of activity, mud volcanoes are grouped in to three main types and the relationship between each type and corresponding morphological expression is discussed. The presented up-to-date data of the world geographical distribution of mud volcanoes show that they approximate to 1800 individuals. A detail overview of geological environments in which they occur helps to deduce some necessary conditions for mud volcano formation. The basic mechanisms of mud volcano formation are discussed, specifically the fluid-pressure hypothesis, and some triggering events are identified. The most common investigation approach and some criteria for recognizing of a submarine mud volcano on side-scan sonar records and seismic sections are given. Mud volcanism in the Mediterranean Ridge (an accretionary complex) and Black Sea Abyssal Plane (a back ark basin with tectonic regime of extension) are described as case studies to show variety in morphology and common factors in development of mud volcanoes. The manifestation of continuous gas migration through mud volcanoes is clarified, and the attempt made to quantify the gas release shows that mud volcanoes are one of the significant natural sources of atmospheric methane emitting a total amount of about 10.3 to 12.6 Tg annually.