Each of us has a store of semantic representations, which code our acquired knowledge about the world and allow us to understand words, objects and events. Effective use of this vast database requires executive control systems that regulate how the information is accessed, so that we focus on the appropriate aspects of knowledge for each situation. For example, if you are searching for a lemon in the kitchen, you will be aided by the knowledge that it is bright yellow. But when you begin to cook with the lemon, you will need to bring information about its flavour to mind instead. My work explores the interplay between the semantic knowledge store and the control processes that regulate it. I will discuss evidence that these two systems have distinct neural correlates and can be independently impaired following brain damage. I will also present recent work indicating that the cognitive control of semantic knowledge declines in old age, even as the size of the knowledge store increases, and that this decline has real-world consequences for older adults’ ability to speak coherently.