This article by Ignacio Montero and Orfelio G. León revises and expands their previously published classification system for research methods in Psychology. The system is organized into three main groups: theoretical studies, empirical quantitative studies, and empirical qualitative studies. Theoretical studies include classical reviews and meta-analyses. Empirical quantitative studies encompass seven types: observational descriptive studies, survey descriptive studies, experiments, quasi-experiments, ex post facto studies, single-case experimental studies, and instrumental studies. Empirical qualitative studies include ethnography, case studies, and action research. The authors provide detailed characteristics and subtypes for each category and illustrate them with actual publications. The classification system is intended to guide researchers in naming their studies and to enhance the reliability of document analysis. The article also discusses the importance of precise information about participant selection and the limitations of mixed methods, which are not included in the primary classification.This article by Ignacio Montero and Orfelio G. León revises and expands their previously published classification system for research methods in Psychology. The system is organized into three main groups: theoretical studies, empirical quantitative studies, and empirical qualitative studies. Theoretical studies include classical reviews and meta-analyses. Empirical quantitative studies encompass seven types: observational descriptive studies, survey descriptive studies, experiments, quasi-experiments, ex post facto studies, single-case experimental studies, and instrumental studies. Empirical qualitative studies include ethnography, case studies, and action research. The authors provide detailed characteristics and subtypes for each category and illustrate them with actual publications. The classification system is intended to guide researchers in naming their studies and to enhance the reliability of document analysis. The article also discusses the importance of precise information about participant selection and the limitations of mixed methods, which are not included in the primary classification.