Recently a great deal of progress has been made in identifying the earliest signs of autism and related autistic spectrum disorders (ASD). This has been made possible largely through prospective studies of high risk infants (all with an older sibling with ASD), in which it has been possible to identify infants suspected of having autism by as early as 12-18 months. This current study will evaluate the effectiveness of a newly-developed parent-mediated intervention paradigm. This intervention protocol is implemented in a home setting and incorporates principles of parent responsiveness training and applied behavioral analysis. The targets of intervention focus on two core domains of development: positive emotion sharing and early social communication. During a period of 6 months, parents will receive intensive training and comprehensive clinical support in order to determine if this intervention changes the developmental trajectory of toddlers considered at high risk for developing autism.