Experimental designs are distinguished as the best method to respond to questions involving causality. The purpose of the present paper is to explicate the logic of experimental design and why it is so vital to questions that demand causal conclusions. In addition, types of internal and external validity threats are discussed. To emphasize the current interest in experimental designs, Evidence-Based Practices (EBP) in medicine, psychology and education are highlighted. Finally, cautionary statements regarding experimental designs are elucidated with examples from the literature. (Contains 2 tables and 4 figures.)