Robots and agents are becoming increasingly prominent in everyday life, taking on a variety of roles, including helpers, coaches, and even social companions. A core requirement for these social agents is the ability to establish and maintain long-term trusting and engaging relationship with their human users. Much research has already been done on the prerequisites for these types of social agents and robots, in affective computing, social computing and affective HCI. A number of disciplines within psychology and the social sciences are also relevant, contributing theories, data and methods relevant for the emerging areas of social robotics, and social computing in general. However, the complexity of the task of designing these social agents, and the diversity of the relevant disciplines, can be overwhelming. This paper presents a summary of a special session at ACII 2009 whose purpose was to provide an overview of the state-of-the-art in social agents and robots, and to explore some of the fundamental questions regarding their development, and the evaluation of their effectiveness.