Published in the Political Science Research and Methods journal, this paper examines the effects of survey mode on patterns of survey response, paying special attention to the conditions under which mode effects are more or less consequential. We use the Youth Participatory Politics survey, a study administered either online or over the phone to 2920 young people. Our results provide consistent evidence of mode effects. The internet sample exhibits higher rates of item non-response and “no opinion” responses, and considerably lower levels of differentiation in the use of rating scales. These differences remain even after accounting for how respondents selected into the mode of survey administration. We demonstrate the substantive implications of mode effects in the context of items measuring political knowledge and racial attitudes. We conclude by discussing the implications of our results for comparing data obtained from surveys conducted with different modes, and for the design and analysis of multi-mode surveys.