This chapter outlines considerations for measuring vocabulary knowledge growth in incidental vocabulary acquisition studies. Several topics are comprehensively described to inform vocabulary assessment design. These include whether researchers should control for previous knowledge of target words, the difference between receptive and productive vocabulary knowledge, and the advantages and disadvantages of using self-assessment of target words. Additionally, offline and online measures are compared, and common methods for collecting online data—including response time, eye-tracking, and self-paced reading—are introduced. Next, the neglected area of the qualitative assessment of incidental vocabulary knowledge is discussed, and issues for future research are raised. Much of the chapter is devoted to the importance of triangulation when assessing the incidental acquisition of target word knowledge. Moreover, a critique of the existing literature outlines the need for studies that take both dimensional and developmental approaches to vocabulary assessment. As the incidental acquisition of vocabulary is often fragile, it is important that researchers obtain more longitudinal data through delayed posttests to measure the retention of incidental vocabulary knowledge. This chapter concludes with a reflective summary of incidental acquisition measurements and future vocabulary assessment designs.