A main concern of street-level bureaucracy research is to explain discretion-as-used. This chapter explores the multitude of theoretical concepts and measurement instruments that have been developed for this dependent variable. Five categories are distinguished related to specific why questions: street-level public servants’ degree of discretion, decisions, interaction styles, coping and compliance. The diversity shown must be embraced for it reflects the fact that many aspects of street-level behaviour are left to varying degrees to the discretion of public servants. Nevertheless, knowledge accumulation hinges on more consistency in theoretical concepts and measurement instruments of the varied aspects of discretion-as-used to foster the comparability of findings across street-level practices and settings. The chapter discusses the theoretical and methodological challenges that researchers encounter when developing well-grounded concepts and instruments. Suggestions are made on how these challenges can be dealt with and what is to be gained.