The authors searched CINAHL, MEDLINE, and Web of Knowledge databases using key terms such as: immunization, parents, parental attitudes, immunization schedule, combined vaccines, immunization/psychology, and immunization/adverse effects. Results were limited to English, and abstracts were read to determine the most relevant articles. Reference lists of retrieved studies were searched to identify further articles related to parental concerns of immune system overload and multiple injections. Twenty-seven articles were chosen, with publication dates ranging from 2000 to 2014. They include both qualitative and quantitative studies, as well as reviews.