Aggression and loneliness are complex, interrelated constructs that influence and reinforce each other across cognitive, emotional, and behavioral levels. Aggression, defined as behavior intended to cause harm, can manifest in overt, relational, or cyber forms and be driven by reactive or proactive motives. It is often preceded by hostile thoughts, anger, and frustration—emotions similarly implicated in the experience of loneliness. This chapter explores the bidirectional association between aggression and loneliness, proposing a conceptual model where both can act as antecedents and consequences of one another. Loneliness may promote aggressive behavior through impaired emotion regulation, frustration of social needs, and disinhibition. Conversely, aggression can exacerbate social rejection and isolation, thereby increasing loneliness. Shared factors such as social withdrawal, hostile attribution biases, reduced self-control, and psychopathological symptoms further complicate their interplay. The chapter emphasizes the need for longitudinal and ecologically valid research to clarify underlying mechanisms and suggests targeted interventions, such as emotion regulation training and relational aggression prevention programs, as promising avenues to disrupt the vicious cycle linking loneliness and aggression.