Objective: High rates of extradyadic involvement during dating have been reported. An increasing attention has been given to extradyadic online interactions. Recently, a measure including a comprehensive list of face-to-face and online extradyadic behaviors (EDB) was developed, the Extradyadic Behavior Inventory (EDBI; Luo, Carter, & Snyder, 2010). The aims of this study were to assess the prevalence rates of face-to-face and online EDB during exclusive dating relationships, and to examine the factor structure of the EDBI. Design and Method: The sample of this cross-sectional study consisted of 947 participants (275 men and 672 women), with a mean age of 23.40 years (SD = 3.99). Participants were in the current relationship for an average of 34 months. Participants completed a self-reported questionnaire on sociodemographic and relational information and the Portuguese version of the Extradyadic Behavior Inventory. Results: Most participants (66.2% of men and 60.4% of women) reported engagement in face-to-face EDB, and 50.2% of men and 42.1% of women reported engagement in online EDB. Men reported significantly higher engagement in online interactions than women. The exploratory factor analysis identified a two-factor structure for both face-to-face (physical/sexual and emotional) and online (sexual and emotional) EDB. Men reported significantly higher scores than women in all forms of extradyadic involvement. Conclusions: The two-factor models identified for face-to-face and online extradyadic involvement are consistent with the perspective that differentiates two types of infidelity – sexual and emotional. By covering a comprehensive list of clearly defined EDB, the EDBI overcomes important limitations of research on infidelity.