This thesis carries out a conceptual analysis of good faith in the UN Convention on Contracts for the International Sale of Goods (CISG) through a comparative study of the principle in French law, the Shari'a, English law and the US Uniform Commercial Code (UCC). The CISG was developed with the aim of establishing a uniform law to be applied to contracts for the international sale of goods. Nevertheless, one of the fundamental issues affecting the successful application of the Convention is the divergent interpretation and application of its provision on good faith by national courts and arbitral tribunals. This is mainly due to the lack of a harmonised conceptualisation of good faith at an international level under the CISG. Therefore, this thesis addresses this particular issue of the application of a harmonised application of good faith under the Convention. This issue was approached in this thesis through a conceptual study of the principle by using a comparative analysis, as the CISG is the construct of a comparative study. The chosen legal systems for this comparative analysis have had an important impact on the drafting and development of the CISG. In its comparative analysis, this thesis shows that good faith is commonly understood as reasonableness and that its main role across the studied legal systems is as an implied term used to interpret the contract. Upon the application of the comparative results of the study against the context of the CISG, this thesis identifies that a harmonised application of good faith under the Convention is possible. In this regard, it is shown in this thesis that good faith under the CISG amounts to what is considered to be commercially reasonable conduct and that it is measured against the standard of the reasonable person. Also, based on the comparative analysis carried out, this thesis concludes that the common approach towards good faith in the CISG would be to apply it to the interpretation of the Convention. Therefore, this thesis makes a conceptual contribution to the harmonisation of the principle of good faith in the CISG and offers tools for other scholarly work that relates to the concept of good faith and its harmonisation or uniformity.