Purpose: The purpose of this study was to investigate typical Spanish-English speech sound development longitudinally in a group of bilingual preschoolers enrolled in a Head Start Program and to examine the extent to which such development is linked to language proficiency. The study also aimed to identify whether speech development is related cross-linguistically and to improve our understanding of error patterns in this population. Method: Thirty-five bilingual preschool children produced single-word speech samples in Spanish and English both at the beginning of their first and their second year in a Head Start Program. Conversational samples in both languages were also collected at these data points to calculate mean length of utterance in words (MLUw) and thus assess the children's linguistic proficiency. The phonetically transcribed speech samples were compared over time in terms of segmental accuracy measures and error pattern frequencies. Correlation analyses were run to examine the relation between segmental accuracy measures across languages and between speech sound production and MLUw. Results: One-way within-subject analysis of variance revealed significant improvements in accuracy over time in both languages, but not always for cross-linguistically unshared segments, nor for all consonant manner classes. Overall error rates decreased over time in both languages; although, certain error types showed no change. Cross-linguistic interactions were low in both languages. The results also revealed significant cross-linguistic correlations in segmental accuracy between Spanish and English, as well as between MLUw and speech sound production in both languages on a range of measures, with language-specific differences in Year 2 of the Head Start Program, but not in Year 1. Conclusions: This study is the first to document developmental changes in the speech patterns of Spanish-English bilingual preschool children over 1 year. Accuracy rates improved significantly in both languages, suggesting that enhanced exposure to the majority language at school may not impede phonological development in the home language. Bootstrapping effects were particularly pronounced on cross-linguistically shared sounds, which suggests that the same underlying skills are utilized in both languages, whereas language-specific singleton consonants and consonant clusters did not appear to benefit from exposure to the other language. The results also suggest an intricate link between phonological skills and morphosyntactic performance at the early stages of development, but a more complex pattern thereafter with differences that may be based on language-specific phonological properties.