Over the current decade, nanotechnology has been most promising research field, showing encouraging applications in the photonics, electronics, medical, and catalyst industries. The nanoforms of metals/metal oxides and nanocomposites make them more effective and efficient tools than their bulky counterparts due to their specific physical, chemical, and optical properties. Recently, nickel oxide nanoparticles (NiO NPs) are gaining much more attention in the different research field due to their unique properties. NiO NPs were synthesized by physical and chemical methods, utilizing toxic chemicals as precursors. NPs so formed also have these toxic chemicals attached to their surfaces, thus reducing their biocompatibility, and making them biologically inactive. NPs synthesized using biological extract like, bacteria, fungi, plant extracts which have biomolecules act as chelating, stabilizing and capping agent, thus, forming bioactive and biocompatible nanomaterials. Therefore, the present review summarizes various green methodologies involved in the synthesis of NiO NPs, typical analytical techniques used for their characterizations, and their potential applications in different fields. Role of NiO NPs in shaping our day-to-day technologies has also been touched upon briefly. © 2022 Elsevier B.V.