This chapter focuses on the first two steps of the bystander intervention model—noticing and interpreting bullying as a problem. Noticing requires directing attention to an event, yet distinguishing bullying from related behaviors such as aggression or peer conflict can be difficult. The chapter reviews how bullying, sexual harassment, and related behaviors are recognized and interpreted and highlights challenges in measuring these processes. The chapter also examines individual (e.g., age, gender, past experiences with bullying) and contextual (e.g., situational ambiguity, peer group norms) factors that influence whether youth notice bullying and perceive it as problematic, ultimately shaping their likelihood of intervening.