Learning is essential. It allows animals to change circumstances, deal with new situations and adapt to environments. Here, we argue that learning, at behavioral and neural levels, involves efficiency, reflected in metabolic cost reductions. Behaviourally, although multiple solutions to a novel problem may be available, all solutions are not learnt - it is too costly. Furthermore, once a strategy has been selected, it is reinforced producing an efficiency that leads to a maximisation of performance and metabolic cost reductions. Learning can be represented in the brain through many mechanisms; however, if learning is truly efficient, then, all such mechanisms should also be accompanied by a reduction in measurable metabolic costs. By thinking about learning in terms of efficiency, not simply as a descriptive term but rather in terms of metabolic costs, it allows learning to be examined more carefully and provides predictions that can be easily tested (and indeed refuted).