This chapter focuses on measuring smartphone addiction (or problematic use). In light of the conceptual issues discussed in Chap. 2 , it is unsurprising that difficulties emerge with translating these into measurement scales. As a result, nearly one hundred questionnaires have been developed to measure problematic or addictive phone use. In addition, a significant minority of studies develop ad-hoc scales or adjustments. Despite, or perhaps because of the volume of scale development research, there are substantial gaps in instrument validity, especially when comparing between different groups. These flaws place significant doubts on attempts to use these scales to classify people as meeting criteria for smartphone addiction, especially when these classify large sections of the population as at-risk. Finally, these issues crystallise when smartphone addiction scales are used in statistical modelling. A combination of poor practices and flexible modelling creates a substantial risk of bias. These measurement issues represent the most substantive threat to the validity of empirical research on smartphone addiction.