This chapter focuses on the influence of environmental factors, especially instruction. It provides some of the more compelling reasons for believing that intelligence is changeable as a consequence of environmental factors. Beliefs, especially about intelligence, can have large effects, both beneficial and detrimental, on cognitive performance. Many researchers have identified working memory capacity as a factor that limits performance on cognitively demanding tasks. There is considerable agreement among many researchers on intelligence that both nature and nurture play major roles in determining intelligence and cognitive performance, despite differences of opinion regarding the relative contributions of the two types of factors. The obvious conclusion is that those who aspire to increase intelligence or to enhance people's ability to perform cognitively demanding tasks, by instruction or other environmental means, are not tilting at windmills but are pursuing a reasonable goal.