This chapter focuses on a particular aspect of the learner of science: the nature and importance of the ideas and beliefs the learner brings to the science classroom. The growth in interest about students' ideas and beliefs is the most obvious feature of considerations of science learning in the last decade. Research has pointed to the common existence of ideas and beliefs before formal science instruction is experienced. These ideas/beliefs are frequently at odds with the ideas of science and can be held to tenaciously by students. This is shown particularly by the relatively common finding that students successful on standard forms of science achievement tests can fail to use this learned science to interpret everyday phenomena and analyze usual situations. Instead the interpretation and analysis are often undertaken with the ideas and beliefs held before encountering the science of the classroom. Data pointing to this are commonly argued to indicate superficial learning and lack of understanding of the content of the curriculum.