Various theories of second language acquisition (SLA) attribute significant importance to the role of input in second language (L2) learning. The current paper attempts to explore the role of input through an overview of four theories of SLA. These theories highlight different views on the value of second language input to learners’ linguistic abilities. These theories are: the Input Hypothesis (Ellis, 1994, Krashen, 1985, 1989, Doughty & Long, 2003; Mackey & Gass 2015), the Input-Interaction-Output Hypothesis (Gass, 1988, 1991, 1994, 1997), the Input and Interaction Hypothesis (Long, 1980, 1985, Brown, 2007, Long, 2016) and the Autonomous Induction Theory (Carroll, 1999, 2001, 2007, Loewen & Sato 2017). Also, the role of interaction for L2 input will be discussed. In fact, learners’ acquisition of L2 depends on different factors, either external or internal as well as on learners’ experience of the language. Therefore, accounts of successful SLA have emphasised the influence of the quality of the input provided to learners, as it counts as one of these external factors. However, input alone cannot facilitate L2 learning, as learners cannot develop full linguistic ability in the target language without processing and practicing the transmitted information through interaction. The present paper is a brief review of the significant role of language input in SLA. Accordingly, the role of input is discussed the perspective of different theories of language learning.