Many students take ACT test twice, once as a junior and again as a senior. You should definitely consider retesting if you had any problems during testing, such as misunderstanding the directions, or feeling ill.
You may also want to consider retesting if you don't believe that your scores accurately represent your abilities, especially if you see a discrepancy between your ACT scores and your high school grades, or if you have subsequently completed coursework in the areas covered by the ACT.
If you test more than once, you determine which set of scores are sent to colleges or scholarship programs. ACT reports scores from only one test date per report.
Research shows that more than 50% of students increase their composite score on the retest. The lower your initial ACT Composite score, the more likely your second score will be higher than the first score. The higher your initial ACT Composite score, the more likely your second score will be the same as or lower than the first score.