This chapter discusses some fundamentals of medical research in detail. The research designs are classified based on the sampling direction, the way in which the study subjects were assembled, and whether the exposure to the intervention or the putative causal factor has been under the control of the investigator or not. In randomized controlled clinical trial (RCT), true random allocations are important for optimizing the likelihood of successful blinding of patients, care givers and assessors. Cohort (non-equivalent) group, concurrent study, avoids having to withhold treatment for those who wish to receive it and is statistically efficient when equal number of exposed and unexposed subjects can be studied. In cross sectional survey several potential causal factors can be compared with several outcomes and the investigator has no influence as to which patients are exposed to the proposed casual factors or interventions.