In the first year of life, infants who later go on to develop autistic spectrum disorders (ASD) may exhibit subtle disruptions in social interest and attention, communication, temperament, and head circumference growth that occur prior to the onset of clinical symptoms. These disruptions may reflect the early course of ASD development and may also contribute to the later development of clinical symptoms through alterations in the child's experience of his or her environment. By age 2, developmental precursors of autism symptoms can be used to diagnose children reliably, and by age 3, the diagnosis is thought to be relatively stable. The downward extension of the autism diagnosis poses important questions for therapists in designing interventions that are applicable for infants who demonstrate early risk factors. We review current knowledge of the early signs of ASD in the infancy period (0–12 months) and the manifestation of symptoms in toddlerhood (12–36 months), noting the importance of considering the variability in onset and trajectory of ASD. Finally, we consider the implications of this emerging research for those who work or interact with young children, including the importance of early monitoring and the development and evaluation of age-appropriate interventions.