A discussion of the relationship of vocabulary to reading comprehension focuses on the instruction of difficult vocabulary to learners of English as a Second Language. The historical background of vocabulary instruction is chronicled and the importance of reading and vocabulary learning to each other is examined. Various kinds of difficult vocabulary are discussed, including unknown words, synophones and synforms, technical and other low-frequency words, false cognates, polysemous words, idioms, compounds, multi-word verbs, and abstract words. Four theoretical approaches to vocabulary acquisition, each suggesting a hypothesis of instruction, are then reviewed. Three of the hypotheses are classified as explicit vocabulary instruction, and the fourth is classified as reading skills instruction. Teaching strategies corresponding to each hypothesis are presented, along with two sample lesson plans, one from explicit vocabulary instruction and the other from reading skills instruction. A 73-item list of references is included. (MSE)