Inducible nitric oxide synthase (iNOS), which is expressed in response to bacterial/proinflammatory stimuli, generates nitric oxide (NO) that provides cytoprotection. Overexpression of iNOS increases the levels of NO, and this increased NO level is implicated in pathophysiology of complex multifactorial diseases like Parkinson's disease, Alzheimer's disease, multiple sclerosis, rheumatoid arthritis, and inflammatory bowel disease. Selective inhibition of iNOS is an effective approach in treatment of such complex diseases. L-Arginine, being a substrate for iNOS, is the natural lead to develop iNOS inhibitors. More than 200 research reports on development of nitric oxide synthase inhibitors by different research groups across the globe have appeared in literature so far. The first review on iNOS, in 2002, discussed the iNOS inhibitors under two classes that is, amino acid and non-amino acid derivatives. Other review articles discussing specific chemical classes of iNOS inhibitors also appeared during last decade. In the present review, all reports on both natural and synthetic iNOS inhibitors, published 2002 onwards, are studied, classified, and discussed to provide comprehensive information on iNOS inhibitors. The synthetic inhibitors are broadly classified into two categories that is, arginine and non-arginine analogs. The latter are further classified into amidines, five-or six-membered heterocyclics, fused cyclics, steroidal type, and chalcones analogs. Structures of the most/significantly potent compounds from each report are provided to know the functional groups important for incurring iNOS inhibitory activity and selectivity. This review is aimed to provide a comprehensive view to the medicinal chemists for rational designing of novel and potent iNOS inhibitors. K E Y W O R D S arginine, inflammation, inhibitors, multifactorial disease, nitric oxide 1 | INTRODUCTION Nitric oxide (NO) is a relaxing factor derived from endothelium, which is responsible for neuronal transmission, cardiovascular, gastrointestinal, genitourinary, respiratory, antipathogenic, and antitumor responses. The production of NO is effected by nitric oxide synthase (NOS) through catalysis of NADPH-dependent oxidation of L-arginine (the substrate). 1NOS exists in three distinct isoforms, depending on the site of its existence that is, neuronal NOS (nNOS, type I), inducible or inflammatory NOS (iNOS, type II) and endothelial NOS (eNOS, type III). Constitutive NOS are regulated by the binding of calcium-calmodulin protein, whereas iNOS is transcriptionally regulated and calcium-calmodulin independent. NO plays both physiological and pathological roles depending on the magnitude and duration of its production. 2 The physiological roles of NO are executed by soluble guanylate cyclase. Indeed, NO activates it, with the consequent increase of cyclic guanosine-3′,5′-monophosphate (cGMP) level, which subsequently activates intracellular effector molecules, cGMPdependent protein kinases, cGMP-gated ion channels, and cGMP-regulated phosphodiesteras...