Word learning is a fundamental element for early language acquisition. A controversial phenomenon associated with vocabulary growth is vocabulary spurt, usually characterized as a rapid increase in productive vocabulary in early child language. Despite the fact that word production initially begins slowly, it has been argued that after a few months children undergo a transition to a later phase of more rapid vocabulary growth (Goldfield & Reznick, 1990). Several theories have attempted to explain this phenomenon. For example, Plunkett (1993, cited in Ganger & Brent, 2004) suggests that acceleration results from linguistic advances such as word segmentation that allows children to pick up more words from the speech stream; however, there is now growing disagreement about its existence in all children (Goldfield & Reznick, 1990; Ganger & Brent, 2004). The purpose of this paper is to evaluate the ability of two theories, namely Artificial Neural Network (ANN) and Dynamic Systems Theory (DST), to explain the issues underlying lexical development and vocabulary development. This essay provides an overview of both theories and compares their strengths and weaknesses in their explanation of lexical development supported by empirical evidence. Both ANN and DST were formed in opposition to the symbolic system view of cognition (Smith & Samuelson, 2003). Although they recognize that some of the underlying mechanisms may be innate, they view lexical development as an emergent process resulting from early social interaction and exposure to linguistic input (Poveda & Vellido, 2006). The main purpose of ANN is to build computational models of various cognitive processes based on biological details of n...... half of the article...... Vellido, A. (2006). Neural network models for language acquisition: A brief survey. In Intelligent data engineering and machine learning – IDEAL 2006 (pp. 1346-1357). Springer Berlin Heidelberg.Robinson, B.F., & Mervis, C.B. (1998). Untangling early language development: Modeling lexical and grammatical acquisition using and extending case study methodology. Developmental Psychology, 34, 363-375. Smith, L. B., & Samuelson, L. K. (2003). Differing is good: connectionism and dynamic systems theory are complementaryemergent approaches to development. Developmental Sciences, 6, 434-439.Van Geert, P. (1991). A dynamic systems model of cognitive and linguistic growth. Psychological Review, 98, 3-53.Van Geert, P. (2008). The dynamic systems approach in the study of L1 and L2 acquisition: an introduction. The newspaper of the modern language, 92, 179-199.