Social and behavioral signals carry invaluable information regarding how audiences perceive the multimedia content. Assessing the responses from the audience, we can generate tags, summaries and other forms of metadata for multimedia representation and indexing. Tags are a form of metadata which enables a retrieval system to find and re-find the content of interest (Larson et al. (2011)). Unlike classic tagging schemes where users direct input is needed, Implicit Human-Centered Tagging (IHCT) was proposed (Pantic and Vinciarelli (2009)) to generate tags without any specific input or effort from users. Translating the behavioral responses into tags results in "implicit" tags since there is no need for users' direct input as reactions to multimedia are displayed spontaneously (Soleymani and Pantic (2012)). User generated explicit tags are not always assigned with the intention of describing the content and might be given to promote the users themselves (Pantic and Vinciarelli (2009)). Implicit tags have the advantage of being detected for a certain goal relevant to a given application. For example, an online radio is interested in the mood of its songs can assess listeners emotions; a marketing company is interested in assessing in the success of its video advertisements. It is also worth mentioning that implicit tags can be a complementary a iBUG group, Imperial College London, UK. His work is supported by the European Research Council under the FP7 Marie Curie Intra-European Fellowship: Emotional continuous tagging using spontaneous behavior (EmoTag).