The Quantitative Modelling Biology Undergraduate Assessment (QM BUGS Version II) assesses undergraduate biology students’ quantitative modelling abilities and confidence. The assessment is intended to be given in undergraduate biology courses where instructors are engaging students in quantitative modelling within biological contexts. The assessment consists of 36 questions: 25 multiple choice questions addressing four subcategories within quantitative modelling understanding (Quantitative Act, Quantitative Interpretation, Quantitative Modelling, and Meta-Modelling) and 11 Likert questions addressing student confidence about modelling in biology within the four subcategories. QM BUGS assessments were piloted in multiple undergraduate biology courses at both a research intensive university and regional university in fall 2017 (QM BUGS I) and spring 2018 (QM BUGS II). Here we present the development and theoretical framework for the assessment, focusing upon reliability and validity evidence with respect to measures of student understanding and student confidence following administration of the QM BUGS II.