The “modern synthesis” in evolutionary biology involved agreement upon a core theoretical framework in biology, the collection, integration and popular communication of a variety of different types of empirical data in support of evolution, and rhetorical and institutional transformations in the discipline. This chapter classifies the key senses of “chance” deployed by (selected) major synthesis authors, and discusses whether and why they regard chance as a more or less significant factor in evolution. As illustrated below, the sense of “chance” intended in any particular author’s work is often a matter of context: different authors mean different things by “chance,” and have different contrasts in mind. Nonetheless, we conclude that there is general agreement on core ways in which “chance” is understood, and core ways in which it’s agreed to play a causal role in evolution. Finally, we concur with Gould that there was a “hardening” of the synthesis, and document a pendulum shift from viewing chance as a relatively minor, to rather more significant, and then less significant causal factor in evolution over the course of the period spanning 1920-1950.