Since the discovery that amnesiacs retained certain forms of unconscious learning and memory, implicit memory research has grown immensely over the past several decades. This review discusses two of the most intriguing questions in implicit memory research: how we think it works and why it is important to human behaviour. Using priming as an example, this paper surveys how historic behavioural studies have revealed how implicit memory differs from explicit memory. More recent neuroimaging studies are also discussed. These studies explore the neural correlate of priming and have led to the formation of the sharpening, fatigue, and facilitation neural models of priming. The latter part of this review discusses why humans have evolved with highly flexible explicit memory systems while still retaining inflexible implicit memory systems. Traditionally, researchers have regarded the competitive interaction between implicit and explicit memory systems to be fundamental for essential behaviours like habit learning. However, recently documented collaborative interactions suggest that both implicit and explicit processes are potentially necessary for optimal memory and learning performance. The literature reviewed in this paper reveals how implicit memory research has and continues to challenge our understanding of learning, memory, and the brain systems that underlie them.