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This chapter provides an overview of experimental and quasi-experimental research designs in the context of applied linguistics research. We begin by discussing key concepts, such as validity and reliability. Following this, descriptions of frequently used design types are provided, including pretest–posttest, time-series, Latin-square, repeated measures, and factorial designs. Examples are used to illustrate each design option. We also discuss the advantages and limitations of each approach, with particular emphasis on how careful research design may help control for threats to the internal and external validity of the findings. We end the chapter by summarizing factors that researchers should take into account when designing applied linguistics experiments.