The world offers learners a seemingly infinite number of word-to-world mappings (Quine, 1960). In order to account for how learners manage to accomplish such a difficult task, theories of word learning have proposed different tools that make the task of learning words easier. However, we propose that reducing difficulty may be detrimental—difficulty may promote long-term word learning. We tested this hypothesis in a cross-situational paradigm in which object-label mappings were ambiguous during each learning event. The three conditions of learning (2 x 2, 3 x 3, and 4 x 4) varied in the degree of difficulty. Results revealed that, although difficulty deterred immediate performance, difficulty promoted long-term performance. We suggest that theory and research should shift from focusing on in-the-moment learning to examining both immediate and long-term learning. A complete theory of word learning not only accounts for word learning in the moment and on each time scale, but also integrates them in order to understand how they influence each other over time.