This research study investigates the development of middle school students’ emerging expressions of uncertainty through observation of 14- to 15-year-olds, challenged in informal inferential reasoning. This study focuses on students’ investigations when sampling from populations and using information from the samples to draw conclusions about the parent populations. The results suggest that when the students engaged in processes of drawing generalised conclusions from data, involving generalising beyond data and using data as evidence of the generalisation, they developed probabilistic language to articulate the degree of certainty embedded in the generalisation. As the students engaged in their inquiries, they developed more sophisticated expressions of the probabilistic language. Attending to students’ emerging articulations of uncertainty when making judgments about the underlying structure of the data and observing patterns and trends in data, provides an opportunity to develop more sophisticated understandings of the developmental process of students’ statistical inferential reasoning.