IntroductionIt is agreed that knowledge of second language acquisition (SLA) contributes to better and effective second language (L2) teaching. However, the main question of how a teacher's knowledge about SLA can enhance or improve his or her teaching remains debatable since there are no theories, approaches or methods that perfectly fit all teaching situations. SLA is “multifaceted in nature” and no single theory will be able to explain how it occurs (Ellis, 1989, cited in Vanpatten & Cadierno, 1993, p. 45). Therefore, for L2 teaching to be effective, a teacher must not simply adopt any theory, approach or method available, but must be able to adapt knowledge to ensure that students get the most from his or her teaching. This essay will discuss two theories important in SLA, mainly the input hypothesis and the comprehensible output hypothesis and how knowledge of these theories contributes to effective L2 teaching in a classroom context. Effective Second Language Teaching Teaching and learning are closely linked, so they are language acquisition and teaching (Liyong, 2006). Ellis (1994, cited in Liyong, 2006) argues that L2 teaching in the L2 classroom will increase L2 acquisition. However, students will never learn everything the teacher wants them to learn. They may notice certain things and ignore others. Even if they pay attention to some things that interest them, this does not guarantee that they are learning or remembering them. Similarly, in terms of L2 teaching, it is impossible for a teacher to teach everything about a target language in a classroom context and even if the teacher manages to teach everything, it is at least... half a paper... .. .ie Linguistics, 2(1). 1-21.Pusat Perkembangan Kurikulum Kementerian Pendidikan Malaysia. (2000). Sukatan Pelajaran Bersepadu Sekolah Menengah: Bahasa Inggeris. Kuala Lumpur.Swain, M. (1985). Communicative competence: some understandable input and output roles in its development. In S. Gass and C. Madden (eds.) Input in second language acquisition. 235–53. Rowley, Massachusetts: Newbury House.Swain, M. (1993). The output hypothesis: speaking and writing are not enough. Canadian Modern Language Review, 50(1), 158-164. Swain, M. and Lapkin, S. (1995) Problems in output and the cognitive processes they generate: a step towards second language learning. Applied Linguistics 16(3), 371–91. VanPatten, B., & Cadierno, T. (1993). Input processing and second language acquisition: A role for instruction. The newspaper of the modern language, 77(1), 45-57.