Purpose: The purpose of this study was to (a) provide evidence for a theoretical model of between-language interaction in bilingual phonological production through the examination of substitution error patterns and to (b) provide developmental data on bilingual children with and without speech sound impairments for use in clinical assessment and diagnosis. Through the lens of markedness, or relative featural complexity, patterns of between-language interaction were observed to provide a foundation for clinical decision making in phonological assessment. Method: Seventy children, ages 3;11-6;7 (years; months), participated in this study: 63 typically developing bilingual Spanish-English-speaking children ([x-bar] = 5;2) and seven bilingual Spanish-English-speaking children with speech sound impairments ([x-bar] = 4;6). Substitution errors in single-word speech samples were analyzed in relation to their language-specific markedness values in terms of both targets avoided and substitutes produced. Both quantitative and descriptive analyses of substitution errors were performed. Results: Bilingual children, regardless of impairment status, abided by the phonological rules of their languages in English and Spanish productions. Findings indicated both typically developing children and children with speech sound impairments preferred the use of unmarked sounds that are shared across languages over the use of marked, language-specific sounds. Conclusions: Through the examination of substitution errors, evidence of between-language interaction and recognition of relative complexity emerged. These results have implications for clinical assessment and diagnosis of speech sound impairments in bilingual children.