This study examined middle and secondary mathematics teachers' knowledge structures and informal inferential reasoning (IIR). Using task-based clinical interviews (Goldin, 1997) and cross-case analysis, nine teachers responded to four LOCUS assessment tasks (Jacobbe, 2016). Responses were used to construct knowledge structure maps for measures of center, spread, and shape (Groth & Bergner, 2013). Teachers' IIR was analyzed for the appropriateness of responses (Means & Voss, 1996) and key components of IIR were identified. Teachers with more connected knowledge structures and fewer undesirable knowledge elements exhibited more acceptable forms of IIR. Although teachers engaged in the inference and data components of IIR (Makar & Rubin, 2009), they rarely referenced uncertainty. Implications for teacher education and future research are discussed. [For the complete proceedings, see ED606531.]