Listening comprehension is viewed theoretically as an active process in which individuals focus on selected aspects of aural input, construct meaning from passages, and relate what they hear to existing knowledge. This theoretical view has not been sufficiently supported by direct research which clarifies what listeners actually do while engaged in listening tasks. This study focused on the mental processes second language learners use in listening comprehension, the strategies they use in different phases of comprehension, and the differences in strategy use between students designated by their teachers as effective and ineffective listeners. The students in this study were all from Hispanic backgrounds, intermediate in English proficiency, and were enrolled in ESL classes at the secondary level. Data were collected using think-aloud procedures in which students were interrupted during a listening comprehension activity and asked to indicate what they were thinking. Findings indicated that mental processes students use in listening comprehension paralleled three thoeretically-derived phases of the comprehension process: perceptual processing, parsing, and utilization. Each phase was characterized by active processing and by the use of learning strategies. Three predominant strategies which differentiated effective from ineffective listeners were selfmonitoring, elaboration, and inferencing. The findings were related to implications for instructional practice.