Nanoparticle (NP)-based analytical methods have displayed a rapid development at the interface of analytical chemistry, food chemistry, biochemistry, and nanotechnology, together with their related industries. For the design of novel antioxidant assays, NPs can be used as colorimetric or electrochemical probes, components in chemical and biological detectors, and radical generation systems. Most applications of NPs used as probes for food chemicals and biochemicals are associated with the use of Au, Ag, magnetite (Fe3O4) or titania (TiO2) nanoparticles and quantum dots. Chemical reduction-based nanotechnological colorimetric assays of antioxidant capacity make use of the formation or enlargement of noble metal nanoparticles (AuNPs, AgNPs, etc.) upon reaction of Au(III) or Ag(I) salts with antioxidant compounds acting as chemical reductants. In this chapter, NP-based methods for the measurement of total antioxidant capacity involving chemical reduction together with the methods for the detection of reactive oxygen and nitrogen species (ROS/RNS) and determination of their scavenging activity have been reviewed. Within this scope, spectroscopic methods associated with electron transfer and noble metal nanoparticles as well as electroanalytical biosensor-originated antioxidant activity/capacity methods using nanostructures have been evaluated. On the other hand, methods with different mechanisms for reactive species estimation, NP-based methods for the detection of hydrogen peroxide and its scavengers, and limitations of NP-based antioxidant assays have also been discussed.