This study develops the Tourism Penetration Index and applies it to 51 islands.  The results confirm the most penetrated comprise Caribbean, Western Mediterranean and Northern Pacific islands typified by large resorts, crowding, short stays and the replacement of man-made attractions for lost amenities.  The least penetrated islands comprise mainly Pacific and Indian Ocean destinations characterized by small facilities, long stays, and limited infrastructure.  The intermediate islands primarily fall into two groups: Caribbean islands advancing to the high-density stage, and other destinations experiencing rapid growth and resource conflicts.  The study concludes with planning implications and suggestions for further research.  Key Words: tourism development, penetration index, islands, world.