Underachievement and overachievement are much more elusive concepts than they appear to be. The reasons are, first, that they are based on perceptions of a difference between ability and achievement, and our measures of both are extremely limited and often deceptive; and second, that they are perceptions, and perceptions are in the eye of the beholder. IQ tests and their proxies are incomplete measures of intelligence (Sternberg, 1994). They measure only general cognitive ability plus some other cognitive abilities, but do not measure broader abilities, such as creative, practical, social, and wisdom-related intelligence. Achievement tests are also very narrow. Many students excel in areas that are not measured by these tests, such as athletics, music, and art. Students may decide their future lies elsewhere from what achievement tests measure and thus that they would rather optimize on other types of accomplishments. Finally, both constructs are perceptions, and different audiences have different perceptions. Because the constructs are person x task x situation x audience interactions, they are specific in applications and limited in value. Gifted underachievement varies along two dimensions: endogenous (internally generated) and exogenous (externally generated), and intentional and unintentional. “Effectivity” is a more useful construct than underachievement or overachievement and I discuss this concept in this chapter. It is one’s effectiveness in getting done what one values and that has prosocial value of some kind.