The three case studies, drawing on qualitative analysis of surveys, interviews, program artifacts, and classroom observation, describe secondary science preservice teachers' evolving expertise at providing opportunities for linguistically diverse students to learn and demonstrate what they have learned (i.e., equitable assessment). The teachers became more knowledgeable about the role of language in assessment and incorporated scientific discourse while assessing in their teaching practicum. Yet, two tensions emerged, which inform the preparation of future science teachers: (a) should language demands of science assessment be reduced or scaffolded and (b) should language use in science as well as scientific understanding be assessed? (Contains 2 tables.)