## 1 Introduction  
Human-robot interaction (HRI) can vary depending on the specific robotic function. In fact, robots have been effectively used in the last decade in therapy and educational interventions, but they have been also used where human interaction is not required, such as for example in robotic vacuum cleaner applications. Even though the latter were not designed to interact socially and form bonds with humans, it was found that people often attribute human-like characteristics to their robotic technology and that they express attachment toward them [1]. This is in line with the assertion that humans have a natural tendency to anthropomorphize everything around them, including technology [2]. This goes especially with children, who \"are unlikely to only use a robot as a tool, and they will undoubtedly have some sort of interaction that can be considered social\" [3]. Therefore, it would be desirable that robots, which are designed to interact with adults and children, would be able to socially interact. To facilitate the interaction, robots should be easy to use, and they should be built to understand and correctly respond to different needs. As robots gain utility, and thereby influence on society, research in HRI is becoming increasingly important. HRI primarily deals with social robotics and includesrelevant aspects of robot action, perception, and cognition. The development of social robots focuses on the design of living machines that humans would perceive as realistic, effective, communicative, and cooperative [4]. For this purpose, social robots should be able to express, through their shapes and behaviors, a certain degree of \"intelligence\" [5]. This skill entails the whole set of social and perceptual abilities of the robot, delivering a human-like interaction. However, on the other hand, the matter about the anthropomorphism of social robots is still strongly debated. Indeed, the inclination to anthropomorphize the existing technology could have negative consequences [6] and even lead to an unrecognized change in human relationships [7] or invasion of privacy [8]. Therefore, it would be prudent to distinguish cases in which the use of anthropomorphism can be encouraged and cases in which it would be better not to [9]. [PERSON] [10] argues that anthropomorphic framing is desirable where it enhances the function of the technology. Especially, it should be encouraged for social robots and discouraged for robots that are not designed in an intrinsically social way [9; 10]. The present review is focused on social robots and in this context, their anthropomorphism is aimed at facilitating the interaction with humans. An important key to reproduce a human-like behavior and facilitate HRI is the understanding of human emotional reactions. In recent years, a great effort was devoted to endowing the robot with the capability of interpreting and adapting to humans' emotional state. Consequently, a core aspect of HRI is Affective Computing. The term Affective Computing was created over 10 years ago; it is defined as \"computing that relates to, arises from, or deliberately influences emotion or other affective phenomena\" [11]. Since then, the community developed different concepts, models, frameworks, and demo applications for affective systems that are able to recognize, interpret, process, and simulate human feelings and emotions. However, these activities have been currently limited to possibility studies, proof-of-concept prototypes, or demonstrators. Human emotions are indeed manifested as visible changes in facial expressions, gestures, body movements, or voice tone [12]. Beyond these observable expressive channels, physiological modulations also occur and can be observed as modifications in blood pressure, heart rate, or electrodermal activity [13; 14]. These modulations are controlled by the autonomic nervous system and are not directly visible to humans.