This essay will provide the language teacher with an introduction to the theory behind the challenges and problems that learners from a variety of language backgrounds face as they learn to pronounce the sounds of English. The primary focus will be on the influence of the first language in second language phonological acquisition. This will include an overview of the role of perception of non-native sounds, as well as a consideration of phonological representation in the mental lexicon and articulatory constraints, all of which can have an effect on difficulties that learners encounter as they learn to pronounce English sounds. Attention will be given to the various components that make up the phonological system, including segmental, suprasegmental, phonotactics, and phonological processes. This theoretical understanding will then be applied to pronunciation instruction within the classroom by addressing what teachers can do to maximize the effectiveness of instruction.