This chapter is concerned with quantitative research designs wherein the objective is to compare competing educational practices or treatments. Although the emphasis is on randomized experimental research where the researcher actively manipulates the treatments, it also addresses a special class of nonexperimental research in which a naturally occurring “treatment” has already occurred. Not addressed are research studies that explore causal relationships between variables that cannot be manipulated, such as between socioeconomic status and educational attainment. The chapter is written in the belief that experimental research in education too often takes a back seat to less adequate quantitative research designs out of expedience or failure to recognize its ability to provide far stronger evidence as to the relative effectiveness of competing practices.