This dissertation explores Spanish nominal plural formation from a morphophonological perspective. The primary objective is to better understand heritage bilinguals' (HBs') phonological categorization of the morphological element of number in their heritage language. This is done by way of picture-naming elicitation tasks of consonant-final nouns and through comparison with first language, Spanish-dominant speakers and second language learners. In addition to the sociolinguistic factors of linguistic experience and quantity of explicit input, lexical frequency and morphological word class are also assessed. The recorded responses from the 148 participants are coded and submitted to a series of binary logistic regression analyses in IBM SPSS Statistics. It is shown that HBs distinguish between different morphological classes and that this has a prominent role in the pluralization of consonant-final nouns in Spanish. Moreover, the present research details the use of not two but three productive plural markers for HBs in Spanish: -es, -s, Ø. The interface approach adopted in this dissertation is proven to more definitively explain plural formation as it examines the connectedness between phonology, morphology, and the lexicon, thus overcoming previous accounts which focused on the influences of these disciplines in isolation. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]