Prevalence of addictive behaviors is commonly associated with alterations in the brain's reward circuitry, leading to the upholding of a maladaptive pattern of reward-seeking behavior despite aversive consequences. The main objective of this systematic review was to provide a broad overview of the literature on electrophysiological correlates of anticipation and reward processing in addiction, including substance use disorders (SUD), as well as behavioral addictions. Reward processing in addiction has been investigated using electroencephalography (EEG) studies employing a cue-reward experimental design, with event-related potentials (ERPs) as the outcome measure. Relevant studies were identified by searching scientific databases: PubMed, APA PsycINFO, Web of Science, and Scopus in April 2026. The terms used to search the databases covered EEG/ERP markers of reward processing in substance and behavioral addictions. The inclusion criteria comprised peer-reviewed ERP studies employing a cue-reward paradigm in adults with addictive behaviors and healthy controls, published in English between 2000 and 2026. Forty-two studies were reviewed (SUD: 23; behavioral addictions: 19). Reward anticipation was indexed by cue-P300, contingent negative variation (CNV), and stimulus-preceding negativity (SPN). Reward processing was examined in distinct early (feedback-related negativity (FRN) and reward positivity (RewP)) and late (P300 and late positive potential (LPP)) reward outcome phases. Altered neural responses were observed across multiple stages of reward processing in both substance and behavioral addictions, consistent with disruptions in the generation, evaluation, and updating of reward expectations. Despite these convergent patterns, methodological heterogeneity limits the strength of current conclusions and highlights the need for greater standardization in future research.