Naturally fractured reservoirs are found in most major hydrocarbon provinces of the world including the Middle East, North Africa, North and South America, and the North Sea. They are generally defined as reservoirs where the flow of hydrocarbons is influenced by an interconnected system of macroscopic planar discontinuities created by natural processes (e.g., tectonic forces or physical diagenesis). These reservoirs present special challenges for development and exploitation and require an understanding of the interactions between the fractures and the background reservoir rock (i.e., the matrix). The geomechanical and reservoir properties of both fractures and rock matrix can vary widely, and one must recognize and account for these properties on a case-by-case basis. In general, enhancement of permeability is usually attributed to the fracture networks while the matrix is associated with storage capacity. But there are exceptions to this general rule, and in highly crystalline rocks, fractures may provide a majority of the porosity. The behavior of fracture systems over the life of a field can lead to prolific producers or disappointing underperformers.