This chapter introduces the main concepts that are discussed in the book: communicative efficiency, costs, benefits and accessibility. It is also proposed that language users’ behaviour is guided by the Principle of Communicative Efficiency. In addition, several more specific principles and heuristics are formulated, which may explain how everyday communication can be efficient. These are three main principles: the principle of positive correlation between benefits and costs, the principle of negative correlation between accessibility and costs, and the principle of maximization of accessibility. The principles work as heuristics that make production and comprehension automatic and therefore more efficient.