Early smiles are a prototypical expression of joy and a window on the development of positive emotion. Smiles elicit positive emotion and engagement in others, a process that contributes to the development of joy and social competence in the young child. Infants express different intensities and qualities of positive emotion through alterations in the temporal and facial dynamics of their smiling and through the incorporation of other expressive actions such as laughter and jumping up and down. Through the first two years of life, infant smiles and laughter become increasingly social and affectively intense, and increasingly used in referential communications about objects; between 2 and 4 years, smiles reflect the social structure of peer interactions. Difficulties with smiling in early interactions reflect a variety of risk conditions, while emotionally positive and responsive interactions can index optimal developmental trajectories.