Sulfur Mustard (SM) is a potent toxic alkylating agent that has been used as a chemical warfare gas during the World War I and in the Iran-Iraq conflict between 1983 and 1988(1). SM can cause serious organ damages especially ocular, neurologic, cutaneous, bone marrow, and pulmonary complications (1). The previous studies have shown that the respiratory complications are the most common late complications of SM toxic exposure including chronic obstructive pulmonary disease (COPD), chronic bronchitis, bronchiolitis oblitrans, bronchiectasis, airway hyperresponsiveness, and lung fibrosis (2-6).The COPD which occur after SM exposure is known as "Mustard lung" (7 ). Since about 45000 patients are now suffering from long term complications of SM toxic exposure, the evaluation of its pathogenesis and finding the possible ways for treatment is necessary. During the last decade, especial attention to the possible underlying mechanism of COPD due to SM intoxications has been applied. Our previous studies have shown that in COPD patients due to SM exposure inflammatory markers (highly sensitive CRP, interleukin 6) are elevated and these markers have direct association with the severity of disease (8,9). The finding which recommends the role of systemic inflammation in the pathogenesis of COPD due to SM intoxication like the COPD due to other causes. In this chapter the historical points, probable pathogenesis, clinical manifestation, and diagnosis of mustard lung will be discussed.