Inducible nitric oxide synthase (iNOS) is one of three key enzymes generating nitric oxide (NO) from the amino acid L-arginine. iNOS-derived NO plays an important role in numerous physiological and pathophysiological conditions, e.g. blood pressure regulation, inflammation, infection, and the onset and progression of malignant diseases. iNOS has been conjectured both as a marker and a therapeutic target in these situations. iNOS is a mediator of unspecific host defence, central in the clearance of bacterial, viral, fungal and parasitic infections. However, excess production of NO appears to be linked to tissue damage and organ dysfunction, e.g. the hypotensive and vasoplegic state characteristic for septic shock. However, the use of iNOS-inhibitors in septic patients should be performed carefully with regard to the essential functions and properties of NO in blood pressure/blood flow regulation. Considering iNOS-derived NO as a multifactorial transmitter of tumorigenesis and tumor progression, it is tempting to speculate on therapeutical interference with iNOS activity, especially in tumors where metastatic activity, host denfence mechanisms and the level of differentiation seem to be correlated to iNOS expression. It is the aim of this review to provide basic insights into the NOS family of enzymes as well as their regulation. In the second part of the review, we will point out the pivotal roles NOS play in inflammation and neoplastic diseases.