This thesis investigates estrogens' modulation of three aspects of social cognition (aggression and agonistic behaviour, social learning, and social recognition). Sex-typical agonistic behaviour (males: overt attacks, females: more subtle dominance behaviours) was increased in gonadectomized mice by estrogen receptor alpha (ERα) agonist 1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT), while non-overt agonistic behaviour was increased in male and female gonadally intact mice by ERβ agonist 7-Bromo-2-(4-hydroxyphenyl)-1,3-benzoxazol-5-ol (WAY-200070). Estrogens also affected the social transmission of food preferences (STFP). Acute estrogen and ERβ agonists WAY-200070 and 2,3-bis(4-hydroxyphenyl)propionitrile (DPN) prolonged the preference for the demonstrated food when administered pre-acquisition, likely by affecting motivation or the nature of the social interaction, while acute PPT blocked the STFP. All mice receiving any of the three treatments chronically showed a prolonged demonstrated food preference, suggesting a loss of ER specificity. Individual differences in social recognition may relate to increased oxytocin (OT) and vasopressin (AVP) mRNA, and ERα and ERβ gene activation, in the medial preoptic area, and decreased mRNA for ERs, OT receptor (OTR), AVP and AVP receptors 1a and 1b in the lateral amygdala. Additionally, dorsolateral septum ERs, progesterone receptor, and OTR may relate to social interest without affecting social recognition. Our and others' results suggest that estrogens, OT and AVP are all involved in social behaviours and mediate social recognition, social learning, social interactions, and aggression. ERs differently modulate the two types of social learning investigated here: ERα is critical for social recognition, but impairs social learning, while ERβ is less important in social recognition, and prolongs the demonstrated food preference in the STFP. This may be due to differences in receptor brain distributions or in downstream neurochemical systems that mediate these behaviours. The results of this thesis suggest that estrogens, through the various systems they modulate, have a key role to play in social behaviour. Further investigations of how estrogens effect change in these systems at the molecular and cellular level, as well as the critical brain areas and downstream effectors involved in these complex behaviours, are needed, and could contribute to therapeutic interventions in socially-based, sexually dimorphic disorders, like the autism spectrum disorders, and women receiving hormone replacement therapy for negative peri- or post-menopausal symptoms. / National Science and Engineering Research Council (PGS-D, CGS-M)