Alice is a 14-year-old high school student, and in her science classes she has been taught quite a lot about the scientific concept of energy. Prior to these lessons Alice certainly used the word “energy” in her every day speech, whether in talking about “having no energy,” referring to the “high energy music” of her favorite band, or trying to reduce “energy consumption” to preserve the environment. During the lessons, Alice struggled to come to terms with some of the scientific ideas, which often seemed to go against common sense. Indeed, her teacher had warned that “this is always a difficult topic to get hold of.” Nevertheless, by the end of the teaching, Alice (who is a bright girl) was able to use the idea of energy in answering questions about batteries and bulbs, chemical reactions, and photosynthesis. However, she still struggled, for example, to see how the products of an exothermic chemical reaction could have the same mass as the reactants, even though “energy has been transferred to the surroundings,” and it didn’t make sense to her that a soda can on her desk “has gravitational potential energy,” even though it “just sits there.” It is clear that Alice has learned something about the concept of energy. How might we conceptualize what has happened to Alice in these particular lessons? What do we mean when we say that Alice has “learned something about energy”? What factors act to influence her learning? What happens to Alice’s existing ideas about energy being consumed, in the face of her new learning? Why should she find some of the scientific energy ideas strange and difficult to understand? There are many questions that might be posed about any such learning event. The aim of this chapter is to review the different approaches taken to characterizing science concept learning. We begin by providing a brief historical overview of