This chapter examines ageism as a sociopsychological construct by integrating theoretical perspectives and empirical findings to illustrate its complexity and varied manifestations across life domains. We propose a contemporary definition that encompasses stereotypes, prejudices, and discriminatory practices directed at others or toward oneself based on age. Building on this definition, the chapter distinguishes three levels of ageism expression, intrasubjective, intersubjective, and institutional, and outlines theoretical frameworks that explain how ageism emerges and is reproduced at each level. The chapter addresses several central debates in the field, including the conceptualization of age as a social construct that underpins social stratification, intergenerational tensions arising in aging societies, critiques of generational taxonomies, and the research bias that prioritizes older adults while neglecting younger cohorts, among other significant issues. Taken together, these reflections provide a conceptual framework and practical guidance for formulating research questions, conducting empirical studies, and proposing interventions to address ageism.