Since the early 1990s, interest into the biological interaction of nanosized particles of various compositions has increased. Following the initial findings that nanoscaled particles can elicit an adverse biological response when compared to their larger (micron-scale) material counterparts, interest into how nanosized materials may elicit potentially adverse effects upon any biological system has been intensively investigated. Over the past 20 years, hundreds to thousands of research studies have been published highlighting the biological effects and interaction of the plethora of nanoparticles (NPs) that are being either accidentally or intentionally (engineered) produced. While a definitive knowledge of many aspects is required prior to investigating the biological interaction of NPs, such as the relevant exposure route to the biological system, the specific characteristics of the NPs being studied, and the realistic dose (concentration) that would interact with the biological system, understanding how the NPs affect the biological system is not based upon any defined theory. In fact, there is no specific understanding as to why particles show different effects when occurring within a certain nanosize range compared to their larger counterpart (micron size range). Despite this, certain paradigms and theories have been proposed and are studied, such as the fiber paradigm and theory of genotoxicity, in order to try and understand such nanoscale effects. The most studied and widely accepted paradigm, however, is the oxidative stress paradigm. This chapter will provide an insight into this paradigm, how it is perceived, how it is studied, why investigating this paradigm in vitro is advantageous, and how the findings associated with this paradigm can provide an insight into the (potentially adverse) biological interaction of nanoscale objects.