Social engineering is the usage of social manipulation and psychological tricks to make the targets assist oﬀenders in their attack. This practice manifests itself in e.g. phishing emails or cold call telephone scams. The aim of the thesis was to investigate the understanding of social engineering attacks in an organisational setting. In particular, the eﬀectiveness both of the threat and the countermeasures was investigated. Tree kinds of social engineering experiments were performed, each using a diﬀerent modality (i.e. Face-to-Face (F2F), email and telephone). In each experiment, the targets (i.e. participants) were persuaded to perform actions that contribute to their victimisation. The subjects (N = 162) in the F2F experiment were visited by an offender in their offices and asked them to hand over their office keys. The subjects (N = 593) in the email experiment received a phishing email with the request to provide Personally Identifiable Information (PII). The subjects (N = 92) in the telephone experiment were persuaded to download and execute software from an untrustworthy website. A portion of the participants in both the F2F and telephone experiment received an intervention to reduce victimisation. The result was that 58.62% of those in the F2F experiment complied with the oﬀender, compared to 36.96% who were priorly informed on how to detect and react to social engineering. In the telephone experiment, 40% complied with the oﬀender, compared to 17.2% who received an intervention. Furthermore, 19.3% of those who received a generic phishing email complied, compared to 28.9% that received a spear phishing email. There was no eﬀect of age, sex and using authority on victimisation found, whereas having had an intervention, receiving a spear phishing email and cultural background did have an eﬀect. It is concluded that awareness raising about dangers, characteristics and countermeasures related to social engineering proved to have a significant positive eﬀect on protecting the target. The research also shows that awareness-raising campaigns reduce the vulnerability only in the short term. In phishing emails, the use of a personalised opening sentence increases its success. The results of these experiments allow practitioners to focus awareness campaigns to maximise their eﬀectiveness.