The main aim of this thesis was to investigate a controversial type of problematic behaviour; problematic internet use, and assess whether or not its characteristics resemble traditional types of addictions such as substance dependence and pathological gambling. Problematic internet use is a construct which has created fierce debates amongst researchers. This reflects inconsistencies in the evidence associated with various factors related to it, which have left researchers arguing whether it is a real psychopathological entity and similar to substance-related and addictive disorders or not. This necessitates further research with an emphasis on identifying similarities and differences regarding problematic internet use in relation to markers that have been reliably associated with the development and maintenance of addictive behaviours. Thus, in this thesis I followed a pragmatic approach and comprehensively investigated behavioural and physiological markers of addictive behaviours in the field of problematic internet use. More specifically, emphasis was given to cognitive markers such as decision-making and the physiological function underling them, attentional bias and inhibitory control processes, as these have been implicated as playing a major role in the initiation and maintenance of addictive behaviour. The main research questions were investigated using a sample of internet users whose internet use ranged from non-problematic to problematic and was associated either with generic or specific online applications; Social Networking Sites (SNS). Overall the results of this thesis support the view that problematic internet use resembles substance-related and addictive disorders and offers important implications for its legitimacy.