This chapter is the first of three that make up Part I of this volume. Part I addresses foundational and defining characteristics of ethnomethodology and conversation analysis (EMCA). This chapter discusses the seminal paper “On Formal Structures of Practical Actions”, in which Garfinkel and Sacks proposed that ethnomethodology provided an alternative (what Garfinkel later called an “alternate”) approach to social actions. The chapter highlights key issues in their argument about the indexical properties of natural language and the trouble they have presented for formal analytic disciplines. Garfinkel and Sacks contrast ethnomethodology's treatment of natural language to what they call “constructive analysis” in the social sciences: methodical attempts to substitute analysable proxies such as codes and indexes for indexical expressions. They propose that indexical expressions performed in everyday settings are themselves “rationally” organised, and that such organisation is investigable without need for such proxies. Their argument provides a starting point for appreciating and critically understanding developments in EMCA discussed in the chapters that follow.