This article presents a theoretical review of and methodological guidelines for the study of two key notions in second language acquisition research, complexity and difficulty. The term "complexity" has gained considerable currency over the past decades and has taken on a wide range of meanings. We argue for a more restricted interpretation, focusing exclusively on formal, structural properties of linguistic items. The less employed term "difficulty" in our account refers to the cognitive costs associated with learning and using such items. On the basis of our theoretical definitions, we critically review measures operationalizing these constructs and discuss their strengths, limitations, and potential applicability to second language research, in order to establish a small set of measures to be used routinely in the interest of replicability and knowledge accumulation in the field. In addition, we discuss the relationship between complexity and difficulty and the associated notions of proficiency and development.