A research design is a systematic plan to study a scientific problem. The research problem determines the type of design. Research designs can be qualitative or quantitative; the latter can be experimental (interventional) and non-experimental (observational). Observational studies are further classified into descriptive (such as case reports, case series and cross-sectional studies) and analytical studies (like case-control and cohort studies). Experimental studies compare experimental groups exposed to different treatments or interventions. All these are briefly discussed along with systematic reviews, meta-analysis, qualitative research, basic sciences research, translational and implementation research.