Social-emotional development, psychosocial functioning, and relational experiences have been shown to impact academic achievement and psychological well-being. Social skills have been identified as being particularly critical in promoting mental health and life-long success. Children with well-developed social skills (e.g., sharing, being cooperative, taking responsibility, trying to understand how others are feeling) typically experience positive peer relationships, favorable psychological experiences, and higher levels of academic success than children without these skills. Due to the persistence of social skills deficits as caused by a lack of skill or ability to perform in a classroom setting, children who struggle to navigate social encounters frequently experience problems in school and social maladjustment. Children from culturally and linguistically diverse backgrounds have been identified as potentially experiencing difficulties in obtaining such skills, which may further contribute to poor academic and social outcomes. Early intervention is therefore necessary, but appropriate and valid assessment methods are first essential. Currently there is a lack of research on the screening and assessment of social-emotional and behavioral functioning for students from culturally and linguistically diverse backgrounds, especially for students identified as English language learners (ELLs). ELLs constitute the fastest growing population of culturally and linguistically diverse children and would likely benefit from more support. The "Social Skills Improvement System" (SSIS; Gresham & Elliott, 2008) is one assessment and intervention program that recently has been proposed as particularly promising for identifying social skills deficits in school-aged children. Social skills are measured across the areas of communication, cooperation, assertion, responsibility, empathy, engagement, and self-control. The SSIS also examines externalizing behaviors, bullying, hyperactivity/inattention, internalizing behaviors, autism spectrum behaviors, and levels of academic competence. The validity and usability of the SSIS, however, have yet to be established in Spanish-speaking ELLs across the continuum of English language proficiency (ELP) levels. This study examined the SSIS's validity and reliability using item-response theory for the purpose of determining whether any items on the SSIS demonstrate differential item functioning in Spanish-speaking ELLs across the continuum of English language development. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]