責任保險自十九世紀以來，已從初期著重之被保險人（即加害人自身）保障，發展成受害第三人之權利保護。其後，保護對象又回歸至被保險人，致使被保險人之免責請求權轉化成為保險給付請求權之主要內涵。為實踐保障被保險人豁免對於第三人責任之功能，保險人抗辯義務乃隨之發生。本文檢視當代責任保險制度之發展，發現其功能已從早期之著重損失填補，逐步擴及至權利保護。其次，本文以美國法制發展為立論重心，探討責任保險人抗辯義務之意義、性質與改革。最後，本文剖析我國現況與潛在缺失，參酌美國司法實務見解，建議修訂保險法規與保單條款，期能成為主管機關與相關單位之參考。 Liability insurance originated in the 19th century and has evolved its function from the protection for the insured as a wrongdoer in the initial stage to the protection for the thirdparty victim. Afterwards, while the target of protection has reverted back to the insured, the major content of the liability insurance coverage has been transformed to indemnification against liability. Under such a scenario, the insurer therefore has the duty to defend for the insured against a third party’s liability claims. After reviewing the contemporary evolution and function of liability insurance, this paper observes that the function of liability insurance has developed to cover both loss indemnity and right-protection. It also focuses the development in the United States, to examine the meaning, characteristics and reform relevant to the insurer's duty of defense. Based on the lessons from the US model, it is suggested that a reform in Taiwan shall be undertaken in respect of the legislation revision and the amendment of policy wordings. It is hoped that its recommendation can be the useful reference for the authority and the insurance industry.