Autism Spectrum Condition (ASC) is a highly heritable neurodevelopmental condition, with behavioral symptoms emerging over the first years of life. A full clinical understanding of ASC will require understanding of social-communication behaviors and associated neurobiology, with attention to when ASC-related differences emerge and how they develop over time. Clinicians often cite age 2 as the time when behavioral symptoms are sufficiently crystallized to assign a stable diagnosis of ASC for many children with ASC, but evidence indicates that both brain and behavioral differences emerge much earlier than this. Chapter 1 examines whether the frequency of socially directing vocalizations toward others at 6 and 12 months of age can serve as an early diagnostic marker, finding that it has the potential to predict diagnostic outcomes at age 2. Chapter 2 examines whether rates of canonical babbling differ in 6- and 12-month-old infants later diagnosed with ASC, and whether these differences can be useful early detection markers. Infants later diagnosed with ASC were found to have lower rates of canonical babbling at 12 months. Very little is known about the relationship between early vocalizations (e.g., babbling, crying, laughing) and the developing brain. Chapter 3 investigates the relationship between early vocal behavior and intrinsic functional connectivity in infants 6-24 months old using functional MRI. Vocalizations showed an inconsistent relationship with connectivity between classic language regions, and few relationships with connectivity between brain networks. A large body of work suggests brain structure begins to differ in ASC in this early period, but the exact nature of brain structural differences that persist into childhood and adolescence has been complicated by conflicting findings. Chapter 4 leverages a recently published method for dissociating two regional properties of gray matter – density and volume – to clarify this relationship in a large sample of 6-19 year-olds, finding differences in volume, but not density. Taken together, findings across studies suggest that infant vocalization behaviors have the potential to serve as early diagnostic markers of ASC. These social communication behaviors and the diagnosis of ASC have nuanced relationships with brain structure and function across development.