Both educational researchers and experienced teachers develop what might be called theories of difficulty. A strong theory of difficulty identifies learners' characteristic trouble spots for a particular area of instruction and includes some causal analysis of why they occur toward improved teaching and learning. The literature on learning and development offers numerous ways of understanding conceptual difficulties, as well as recognizing problems of ritualized knowledge, inert knowledge, knowledge too foreign for learners to engage it readily, and tacit knowledge, the partly unconscious nature of which poses learning challenges. In a number of studies, a strong theory of difficulty has led to improved learning. In everyday teaching, teachers' response to recurrent difficulties may fall short. One not uncommon reaction is to blame the learners' weaknesses and simply keep teaching in the same way. Another better reaction is to 'teach harder', lavishing more time and attention on characteristic difficulties without any causal analysis of what makes them problematic. Most effective is to 'teach smarter' based on a causal analysis refined through experience. The construction of informal theories of difficulty is an important part of the craft of teaching.