This thesis explores the language development of early sequential bilingual (ESB) children.This group speak a language other than English at home (L1) and are introduced to English before the age of five years. Although over 20% of school age children in Australia are ESB, there is limited literature investigating typical language development in this group. This presents a unique challenge for speech-language pathologists (SLPs) working with ESB children. Without a benchmark for typical language development, it is difficult to validly discern atypical development and identify language disorder.This thesis has a strong clinical focus, providing essential foundations for Australian SLPs in an area where none currently exist. It is divided into three sections, each addressing a key area of clinical knowledge: understanding typical development, identifying disordered development, and intervention. The findings are considered within a dynamic interactive processing framework of language (Kohnert, , 2013 which considers interactions between language and the environment, as well as interactions between language and other cognitive systems within the individual.Section 1 presents two papers investigating the features of typical L1 and L2 lexical development in Australian ESB children. First, a longitudinal study investigated L1 and L2 lexical development during the initial stages of L2 exposure. Nine Samoan-English ESB children and matched monolingual controls were assessed four times during their first two years at school. Patterns of ESB lexical learning were identified between and within each language. Importantly, this study highlighted composite scoring as a valid methodology for assessing the lexical skills of ESB children.Data from the longitudinal study revealed a second element of language learning in SamoanEnglish bilinguals. Receptive and expressive vocabulary tasks evaluated acquisition of four word types: cognates, matched nouns, phrasal nouns and holonyms. Each word type had varying phonological and conceptual difference between Samoan (L1) and English (L2).Results highlighted conceptual distance between L1 and L2 as a key factor in L2 lexical acquisition. The children acquired L2 lexical items earlier if their conceptual representation was similar to that of L1. Words with greater conceptual distance between L1 and L2 (phrasal nouns and holonyms) emerged more slowly. This suggests that L1 influences L2 lexical acquisition in this population.iii The second section of this thesis investigates how assessment can validly differentiate language difference from disorder in Australian ESB children. Knowledge of typical ESB language development was used as a means to identify disordered language development. A single case series presented the cases of school age sequentially bilingual children. A range of clinically feasible, culturally sensitive assessment techniques were utilised to achieve valid differential diagnosis. The cases suggest two essential standards for evaluating bilingual language developm...