Second Language Acquisition (SLA) research examines how learners develop language proficiency beyond their native tongue. This paper reviews literature and empirical studies on interlanguage theory, focusing on the presence of systematic errors in SLA. Interlanguage, an intermediary language system, displays consistent deviations from both native and target languages. The review highlights the systematic nature of interlanguage errors, influenced by factors such as language transfer and learning strategies. Empirical findings support the existence of interlanguage across diverse language contexts, indicating its role in learners’ developmental stages. Understanding interlanguage is crucial for informing language teaching methods and facilitating effective language acquisition.