The purpose of this quantitative correlational study was to examine (1) the predictive relationships of smartphone addiction, social networking sites usage, and the fear of missing out with academic achievement as measured by GPA in undergraduate students from 18 to 25 years of age using stepwise regression; (2) the correlation between levels of social networking sites usage and smartphone addiction; and (3) the correlation between levels of social networking sites usage and the fear of missing out. The theory of flow and social capital theory provided the foundation for this study. A convenience sample of 135 undergraduate university students completed an online survey that included three validated instruments: Smartphone Addiction Scale-Short Version, Social Media Use Integration Scale, and Fear of Missing Out Scale. Three research questions were aligned with the three objectives specified in the study purpose. The results of the stepwise regression performed to answer the first research question showed no significant predictive relationship between smartphone addiction, social networking sites usage, and the fear of missing out with GPA, F(3, 131) = 1.243, p = 0.297; adj. R[superscript 2] = 0.005. The parametric correlation computed to answer the second research question showed a moderate-high positive relationship between social networking sites usage and smartphone addiction, r(133) = 0.624, p < 0.001. The nonparametric Spearman's rho correlation performed to answer the third question showed a statistically significant, weak positive correlation between fear of missing out and social networking sites usage, r(133) = 0.292, p < 0.001. These results may be relevant to undergraduate university students as well as to administrators, counselors, and all student support personnel interacting with undergraduate university students. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]