預約是一個獨立於本約之債權契約，有其交易上之實際需要及對本約之「預備性功能」，可先確定當事人有受契約拘束及訂立本約之意。預約就是承諾為本約之「訂約行為」或為本約之「訂約之意思表示」，當事人負有訂立本約之義務乃是「預約概念之內在必然」之結果。預約具有獨立強制履行之效力，非僅得請求損害賠償。預約債務人不為意思表示，債權人得訴請履行，法院得以確定判決代替該意思表示。為求訴訟經濟，當事人得以訴之合併同時請求訂立本約與履行本約。預約必須在內容上具有足夠探知本約內容之確定性時，才具備預約之效力。預約有疑問時則透過其高度確定性可認其為本約，以符合預約之例外性格。至於預約因欠缺本約內容之可確定性而無效時，不排除可依締約上過失主張信賴利益損害賠償；但預約有效而不履行時（特別是台灣實務上常見的預約給付遲延與給付不能），則可依債務不履行請求損害賠償。其賠償範圍與本約債務不履行之履行利益相同。

The preliminary contract is an independent contract, which serves the actual needs in transactions and provides ”preparatory functions” to the contract to make sure that the parties agree to be bound and to enter into the formal contract. As the preliminary contract indicates either the acceptance of the contract or the intent to enter into it, that the parties are under obligation to form the contract is certainly the essential effect of the concept of preliminary contract. Therefore, besides the remedy for non-performance to claim for compensation, the content of a preliminary contract can be independently realized through the compulsory execution. If each party conducts a suit for the performance of the preliminary contract, the conclusive judgment of the court can be substituted for the expression of intent of the other party. Furthermore, to meet the demand of ligation economy, the parties can initiate the action and concurrently assert the claims to constitute the formal contract and to perform accordingly.A valid preliminary contract requires its content to be sufficient, and to be adequate for the parties to foresee the content of the formal contract. However, as the preliminary contract is recognized only in exceptional cases, a contract in doubt, in which the content is quite sufficient and certain, should be seen as a formal contract. If the content of a preliminary contract is insufficient and as a result to be invalid, then it is possible for the parties to claim for reliance damages as the remedy for breach of a duty prior to the conclusion of the contract (culpa in contrahendo). And if the preliminary contract is valid but not to be performed (especially in cases related to delay of performance and impossibility of performance, which are quite common in judicial practices in Taiwan), the parties can claim for damages according to the rules of non-performance, and it is just equivalent to the performance interest of the formal contract.