This study aims at explaining the impact of Instagram social media influencers on behavioural intentions (i.e., intentions to interact with them and recommend them, and also purchase the products they endorse or post). It focuses on influencers’ expertise and their role as opinion and taste leaders. Based on opinion leadership theory, a survey was conducted on 210 Instagram users. Main findings reveal that post originality, uniqueness, and quality impact on influencers’ opinion leadership which, in turn, impacts on behavioural intentions. Influencers’ expertise impacts on the intention to recommend the influencers and purchase the products posted, while influencers’ taste leadership only influences the intention to interact with them. Few papers analyse the direct effects of influencers’ expertise and opinion and taste leadership on followers’ behavioural intentions. Consequently, when designing a social media marketing campaign influencers’ selection should be made according to those traits which stimulate specific behaviours such as fostering interaction, increase electronic word of mouth or persuade to purchase the products posted. In case of product endorsements, selecting influencers with acknowledged expertise is recommended as they will cause an impact on the intention to purchase endorsed products. Influencers perceived as opinion leaders have an overall influence on their followers’ behavioural intentions.