This thesis critically analyses the law of promise. It does so for the purposes of identifying potential solutions to practical and doctrinal problems in the Thai law of promise. Scots law is chosen as the main point of comparison because, inter alia, both jurisdictions are mixed jurisdictions. Scots promissory law was influenced by the Canon Law and was part of the ius commune tradition. Scots law was not influenced by English law in this area. Scots law has developed its own promissory obligation as a free standing legal entity outwith contract. Thai promissory legal principles were derived from both Civilian and English sources. Consequently, promissory language is used both in the sense of a unilateral obligation and a contractual promise. Moreover, the Thai drafters did not acknowledge the different attitude towards a unilateral promise of French law (where a promise must be accepted in order to be binding) and German law (where particular types of unilateral obligations are recognised). This thesis argues that the flaws in promissory provisions under the Thai Code stem from the fact that, inter alia, the drafters did not understand the difference between unilateral and bilateral obligations. This thesis argues that the Scots promissory approach presents a more efficient structure of the law of obligations than the Thai approach. It encounters fewer problems than Thai law because a promise is deemed to be a standalone obligation. This thesis further analyses the practical applications of promise, arguing that a promissory analysis is useful in conceptualising practical circumstances. Adopting a promissory approach is beneficial, making doctrinal analysis clearer in comparison with the offer and acceptance approach. This thesis takes into account the role given to promise in the DCFR. The notion of a unilateral undertaking in the DCFR illustrates that the most recent model rule of European private law recognises the importance of a unilateral obligation. This reflects the fact that the notion of a contract cannot appropriately deal with certain situations in which a person unilaterally intends his/her undertaking to be bound without acceptance. It is concluded that the Scots approach of regarding a promise as an independent obligation separate from contract could be adapted to Thai law. There are certain resemblances between Scots and Thai law in promissory theories and the obligational nature of a promise. Therefore, Thai law is not unfamiliar with the notion that a declaration of wills can unilaterally create an obligation. The proposed approach provides a number of advantages e.g. eradicating an overlap between a promise and an offer; clarifying the legal status of promise; and making the legal status of a promise to make a contract compatible with a promise of reward. In particular, this thesis postulates that promise has a substantive role to play in governing an offer specifying a period of acceptance. This particular observation has, to date, not been made in relation to Thai law.