Input is a very significant factor in second language acquisition (SLA), and language learning. Moreover, without the input, the output or the production is not meaningful that is without the input of some sort, the learner cannot communicate effectively. Input processing model is originally proposed by Bill VanPatten, and he explained his model with two main principles, and some sub-principles. While it is believed that input processing theory is not a very comprehensive model in language acquisition in order to explain all of the aspects of second language acquisition, it can provide a better understanding of acquiring any language in any context. it is needed to mention that the basis of the model is according to Krashen. The principles, misunderstandings, claims, and implications of the input processing theory are provided in detail. Afterward, input processing theory is viewed from a dynamic system lens, and further details are discussed in the paper. Discussion and conclusions are provided in the paper.