Bottom-up paradigms prioritize empirical data from which to derive conceptualizations of psychopathology. These paradigms use multivariate statistics to identify syndromes of problems that tend to co-occur plus higher-order groupings such as those designated as internalizing and externalizing. Bottom-up assessment instruments obtain self-ratings and collateral ratings of behavioral, emotional, social, and thought problems and strengths for ages 1½–90+. Ratings of population samples provide norms for syndrome and higher-order scales for each gender, at different ages, rated by different informants, in relation to multicultural norms. The normed assessment instruments operationalize the empirically derived syndromes and higher-order groupings for applications to clinical services, research, and training. Because cross-informant agreement is modest and no single informant provides comprehensive assessment data, software compares ratings by different informants. Top-down paradigms prioritize conceptual representations of the nature and structure of psychopathology, as exemplified by psychodynamic, DSM/ICD, and HiTOP paradigms. Although these paradigms originated with observations, they tend to prioritize conceptual representations over empirical data.