The objective of this chapter is to provide a theoretical background on mixed-mode designs for online research with an empirical knowledge base on the implications of mixed-mode for data integrity, questionnaire design and analysis. Online surveys have a great potential, but they also still have limitations (e.g., coverage, nonresponse). The rapidly growing interest in web surveys, their potential and limitations, gave a new impetus to mixed-mode designs. These designs promise better coverage and higher response, but may lead to problems of data integrity as the goal in mixed-mode studies is to combine data from different sources. This assumes that data can be combined. The chapter discusses central issues for equivalence of measurement, gives an empirical review of existing studies comparing online surveys with traditional data collection methods and focuses on data integrity issues. In addition, attention is paid to questionnaire development for mixed-mode studies. The chapter ends with statistical analysis issues and how to investigate potential mode effects and check for equivalence.