This chapter provides the theoretical foundations for two core aspects of the project design process, namely: research design and sampling. It explores five different research designs: cross-sectional, case study, longitudinal, experimental, and comparative. The chapter focuses on sampling, detailing: probability sampling, non-probability sampling and sampling that can be both probability and non-probability depending on the context. The research designs are different to the research strategies, in the sense that they are not considered to be inherently quantitative, qualitative, or mixed methods. The cross-sectional research design is popular with experienced and student researchers alike, since it offers a straightforward approach to collecting and analysing both quantitative and qualitative data. The longitudinal research design is concerned with the collection and analysis of data over time. The experimental research design ‘is an empirical investigation under controlled conditions designed to examine the relationship between specific factors’.