Autism Spectrum Disorder (ASD) is a lifelong neurodevelopmental condition characterized by persistent challenges in social communication and a restricted range of interests and activities. Each child presents with a unique constellation of strengths and needs, resulting in substantial heterogeneity. In terms of communication, which is the focus of this dissertation, children with ASD have limited social communication skills in general, with some children presenting as minimally verbal while others have age expected spoken language skills. Supporting children to develop effective spoken communication is an important goal of intervention in the preschool years because the ability to produce multiword combinations spontaneously and regularly before the age of 5 is the strongest prognostic indicator for academic achievement and social functioning during the school years. Children who cannot communicate effectively using spoken language are often recommended augmentative and alternative communication (AAC) systems to facilitate their broader communication development. There is a growing body of research documenting the potential for AAC to contribute to communication gains in children with ASD when implemented as part of early intervention in the preschool years. However, communication gains are varied, and there is only limited research evidence regarding the factors that may influence a child's response to AAC interventions. The lack of research evidence creates challenges for clinicians needing to make informed decisions about the use of AAC. Therefore, there is both a clinical and research need to identify factors that may predict how individual children will respond to AAC interventions. The overall objective of this thesis was to identify factors that may predict, mediate, and moderate the response of children with ASD to AAC interventions. The first phase of the project involved a systematic review to identify factors reported in previous research that may predict, mediate, and/or moderate spoken language outcomes for children with ASD receiving AAC. A total of six articles representing seven original studies met the inclusion criteria, revealing 18 different factors. Despite the small number of studies, the factors identified are informative and contribute to a better understanding of individual differences. However, most of the 18 factors were classified as predictors, whereas only nine were classified as mediators or moderators. The relatively small number of studies examining mediators and moderators limits our ability to understand the mechanisms that underlie how AAC interventions promote spoken language outcomes in children with ASD and highlights the need to further investigate the variability in response to AAC intervention. Furthermore, the paucity of research impacts clinicians seeking to make evidence-based decisions. To help address this gap and identify additional mediating and moderating factors, it is important to explore clinicians' insights about the AAC factors that they believe contribute to the variability in spoken language outcomes in this population. The second phase of the research addressed this gap by administering an international online survey to (a) examine speech-language pathologists' (SLPs) knowledge and use of research on factors that may predict, moderate, and mediate spoken language outcomes; and (b) identify additional factors based on clinicians' insights and experience. A total of 187 SLPs were surveyed, with 37.9% reporting that they were very familiar with factors found in research, and 66.1% reporting that they frequently considered these factors when making AAC decisions. These results indicate that most SLPs attempt to acquire and use research evidence. The survey also captured 31 factors from SLPs' clinical observations, 20 of which were not identified in the systematic review conducted in Phase I. These 20 novel factors provide new clinically relevant avenues to explore and could potentially inform how to individualize AAC interventions. The third phase of the research examined the moderating effects of four SLP identified factors: proximity of AAC to the child, AAC instruction strategies, social motivation, and teachers' perception of AAC usefulness. The impact of these factors on spoken language outcomes was examined in a group of 35 minimally verbal children with ASD who were enrolled in an early intervention program across six centers in Australia. Using linear mixed-effects models, the results indicated that the clinician-identified factors were able to explain variability in spoken language outcomes. However, the significant effects of each factor were dependent on another factor in the model. This highlights the importance of examining relationships between moderating factors rather than simply examining bivariate cause-and-effect models. The compilation of results from this thesis contributes new knowledge by refining our understanding of how AAC interventions can potentially be personalized to a specific subgroup of children with ASD who are minimally verbal. The results from this thesis support what has been found in previous research and clinical observations; that is that the response to AAC is variable, and there is a need to tailor AAC to meet the needs and skills of the AAC user. However, the current research evidence is limited on what components of AAC therapy are most effective, what type of providers are most effective, and under what conditions. The compilation of results from this thesis contributes to our understanding of how AAC interventions can be personalized to a specific subgroup of children with ASD who are minimally verbal by highlighting factors that predict, moderate, and mediate AAC outcomes. The collaborative approaches taken between researchers and clinicians were used to identify and explore factors, and provide results that have clinical utility, which ultimately contributes to the development of stronger clinical decisions around individualizing AAC interventions when targeting spoken language.