Social enterprises are attracting a lot of public attention as key contributors in solving some of the most pressing social and environmental issues. In response, many jurisdictions throughout the world have been enacting legislation for the new forms of organizations designed for social enterprise. Several types of hybrid organizations that inject non-profit legal characteristics into their traditional for-profit corporate structures have been developed over the decade, in particular in the US and UK. This article analyzes the theoretical and practical issues of these hybrid organizations and examines whether we should introduce such new forms in Japan. Challenging experiences in the US and UK may offer valuable lessons to Japan when introducing hybrid organizations. In Part I of the article, the relation between the definition of social enterprises and various forms of organizations is addressed. Part II analyzes legislation enabling social entrepreneurs to adopt hybrid forms to house their entities in the US. Experiences in the US may expose some fundamental problems faced in hybrid organizations.