Empathy is generally described as a multidimensional construct, consisting of cognitive and affective components. Researchers demonstrate that a better ability to understand and express empathy toward others is associated with positive social outcomes such as strong communication skills and meaningful social relationships. For individuals with intellectual and developmental disabilities (IDD), the understanding and expression of empathy and its various components can be challenging. However, different etiologies can elucidate various strengths and weaknesses related to empathy. Given empathy's critical role in various social domains and potential differences in empathy skills and social outcomes across different IDD groups, the purpose of this dissertation is to expand the current literature on empathy for three distinct and unique IDD groups to better inform current and future interventions. This dissertation consists of three independent, but related manuscripts presented in journal submission format. The first study was a systematic literature review of 169 studies conducted to identify the current research on empathy skills and social outcomes for individuals with autism spectrum disorder (ASD), Williams syndrome (WS), and Down syndrome (DS). Overall, the results of the systematic literature review indicate that individuals with ASD experience poorer empathy skills and these deficits relate to poor social skills. Further, there is a lack of research on empathy for other IDD conditions (e.g., WS and DS) and in relation to other social domains (e.g., social skills). As such, the second study examined the specific relationship of empathy and social skills for individuals with ASD, WS, and DS and examined for differences across groups and compared to a typically developing (TD) control group. The final sample (N = 120) included 30 students diagnosed with ASD (mean age = 10.73 years) and their caregivers, 30 students diagnosed with WS (mean age = 12.07 years) and their caregivers, 30 students with DS (mean age = 11.53 years) and their caregivers, and 30 TD students (mean age = 10.90 years) and their caregivers. Results demonstrate that, according to parent reports, students with ASD present with lower empathy skills compared to students with WS and DS and to students without disabilities. Differences between groups were also demonstrated regarding the relationship between empathy skills and social skills. More specifically, for students with WS, lower empathy skills were not significantly correlated with social motivation. The results of Chapter 3 highlight that differences in empathy and social skills should be accounted for in empathy-related interventions and underscore the importance of developing etiology-specific interventions. In response to the importance of developing interventions that account for the student's disability, the third manuscript was written to guide special educators in addressing empathy skills for students with ASD. When assessing empathy skills, it is crucial that special educators obtain accounts from multiple informants including parent- and teacher-reports. The use of multiple sources will provide both a general picture of a student's empathy skills along with information for specific areas of concern. Other general considerations for empathy assessment and intervention include incorporating elements of cognitive and affective empathy during assessment, adopting a developmental framework to guide both short term and long-term goals, and identifying additional social skills deficits to target prior to or during the empathy intervention, if applicable. [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.]