This article describes a developmental sequence for students' understandings about conductivity. From written responses, a number of levels of understanding were identified and holistic descriptions of the increasingly complex way students explain conductivity are presented. Identifying distinct differences in student work samples is consistent with current assessment procedures based on levels of achievement. The criteria used for making decisions about the different levels in the sequence are described and also how these decisions were generalized within the framework of the Structure of the Observed Learning Outcome (SOLO) Model. Some professional development implications are identified for school, system and Association levels. (Contains 4 tables and 3 figures.)