證券交易法上對於有價證券的定義與投資人保護有著密切的關係，因為其對有價證券定義，會直接影響其適用的範圍。然而，如何定義和解釋有價證券並不容易，因為商業和經濟活動的多變和多樣性，不法或投機者常會以不同的名目或投資方式來向缺乏專業知識的投資者推銷，藉以逃避證券交易法中對有價證券的定義。另一方面而言，過度的擴張有價證券定義的範團和證券交易法的適用，有時不但無益於增進投資人保護反而會損害其利益。美國案例法對於證券法中對於有價證券的定義，運用投資契約的理論和彈性定義原則，有效因應商業活動的多變和多樣性，來保護投資人，且其亦考量專業投資人的角色和投資人自己責任的投資保障真諦。本文就美國案例法中如何運用投資契約（Investment Contract）的彈性定義原則來保護投資人與其如何將專業投資人的角色和投資人自己責任的概念納入考量進行介紹，並探討投資性契約概念在國內證券交易法中有價證券定義和法院適用時所扮演的角色。

The definition of securities under securities regulation has a close relationship with investor protection since the definition of securities under securities regulation directly determines the scope of the protection for investors. However, defining securities is not an easy task. Because of the dynamic of business world and the limitation of the statutory language, wrong doers by selling various types of the investment products can avoid the application of securities regulation. On the other hand, over extending the application of securities regulation may damage the interests of investors because securities regulation may create an urmecessary complying cost for professional investors This article introduces the concept and the application of the investment contract used by the U.S. court for dealing its securities definition cases. This article explains how the U.S. court by using the Howey Test to find the right balance between the investor protection and the investor's own liability. This article also compares the method to define securities under the securities regulation between the U.S. case law and Taiwanese legal system.