Health psychology is a branch of psychology which studies the psychology of the patient’s behaviour, the role of psychic factors in the origin and development of the diseases, the psychology of relationships between doctor, staff and patient, as well as the use of a psychological approach in their assessment and treatment in medical practice. Nowadays, researchers in Health psychology are interested in applied psychology issues conducting research on how patients are treated in hospital, how social factors influence the behaviour of an individual patient, how to motivate health workers to perform at a higher level etc. These research questions cannot be answered by lab experiments as one has to go to the field and the real life situation like the classroom etc., to find answers to the research issues mentioned above. Thus, the quasi-experimental research came into existence. Quasi-experimental research eliminates the directionality problem because it involves the manipulation of the independent variable. It does not eliminate the problem of confounding variables, however, because it does not involve random assignment to conditions. For these reasons, quasi-experimental research is generally higher in internal validity than correlational studies but lower than true experiments. Quasi-experimental research design can be more easily implemented in natural settings and one can make direct assessment of subjects, find out the effects of a specific treatment introduced by the researcher, and while doing so the researcher can also minimise the influence of extraneous variables. In this paper, the quasi - experimental design is discussed for health psychology.