Research on conceptual change assumes that students enter a science classroom with prior(mis-)conceptions. When being exposed to instruction, students are supposed to develop or change their conceptions to (more) scientific concepts. As a consequence, instruction typically concentrates on appropriate examples demonstrating that students' conceptions are limited and need to be extended or revised ("Posner criteria"). Based on our studies on students' conceptual development in Physics, we rather argue that students typically lack any (explanatory) conceptual understanding of the science content offered. We therefore conclude that a focus on missing conceptions is much more promising than a focus on misconceptions. This paper addresses theoretical arguments and empirical results supporting our proposition as well as suggests possible implications for the design of instruction and for teacher education. (Contains 2 tables, 4 figures, and 5 notes.)