In addition to conventional natural gas, there are several types of unconventional gas resources that are currently produced and these are: (1) methane hydrates—natural gas that occurs at low temperature and high pressure regions such as the sea bed and is made up of a lattice of frozen water, which forms a cage around the methane; (2) biogas, which is a gas produced from various types of biomass; (3) coalbed methane—natural gas that occurs in conjunction with coal seams, coal gas, which is a gas produced by the thermal decomposition or gasification of coal; (4) flue gas, which is a gas from various industrial source that is sent up a flue for dispersal; (5) gas in geopressurized zones—natural underground formations that are under unusually high pressure for their depth; (6) gas in tight formations, which is a gas located in reservoirs in which the permeability is zero or, at best very low; (7) landfill gas, which is a gas produced by the decomposition of landfill materials; (8) manufactured gas, which is a fuel–gas mixture made from other solid, liquid, or gaseous materials, such as coal, coke, oil, or natural gas—examples are retort coal gas, coke oven gas, water gas, carbureted water gas, producer gas, oil gas, reformed natural gas, and reformed propane or liquefied petroleum gas; (9) refinery gas, also called petroleum gas, which is a gas that emanates from the top of a refinery distillation column or from any other refinery process; (10) shale gas, which is a gas that is recovered from shale formation; and (11) synthesis gas, also known as syngas, which is a mixture of carbon monoxide (CO) and hydrogen (H2) and is produced from a wide range of carbonaceous feedstocks. This chapter presents a description of the origin and properties of each of the nonconventional gases.