It is important to examine the underlying factors in the development of smartphone addiction and nomophobia (no-mobile-phone phobia), which can be considered as an extension of it which has become a mental health and social problem by spreading globally and, for every age group. From this point of view this study examines smartphone addiction and nomophobia in the context of attachment styles, sense of identity and interpersonal problems, which are thought to be related. The sample of this cross-sectional study consisted of 532 participants aged between 18-65 years. Smartphone Addiction Scale-Short Version, The Nomophobia Questionnaire, Relationships Scales Questionnaire, Sense of Identity Assessment Form and Inventory of Interpersonal Problems Circumplex Scales were used as measurement tools. The results of the analyses indicated that smartphone addiction and nomophobia scores showed significant differences according to demographic variables. In this study, the risk of smartphone addiction and nomophobia in the adult population sample was calculated as 53.4% and 61.1%, respectively, and it was thought that these findings would give an idea about the prevalence of these phenomena. According to the correlational analysis results, statistically significant correlations were found between smartphone addiction, nomophobia and predictor variables. In addition, it was concluded that smartphone addiction was significantly predicted by secure attachment, preoccupied attachment, sense of identity and interpersonal problems by 25%, and nomophobia was significantly predicted by sense of identity and interpersonal problems by 9%. The findings obtained from the research are discussed in the light of the relevant literature.