Quantitative research uses large samples and, as such, the findings of well-conducted studies can often be generalised to larger populations. However, it is important that studies are well-designed to avoid errors in their interpretation and/or the reporting of inaccurate results. Misleading results from quantitative studies can have serious negative implications such as wasting public money on flawed policies and subjecting service users to ineffective or harmful treatments. This chapter explores descriptive and experimental quantitative research designs and examines, through case examples, the difference between cross-sectional, longitudinal and cohort studies. Factors leading to poorly and well-constructed studies are explored, along with a discussion of the key features of well-designed randomised controlled trials, the gold-standard design for testing treatment effectiveness.