The evaluation of digital public health (DiPH) interventions is as necessary as the evaluation of any other medical or public health intervention. This chapter addresses the two important dimensions of evaluation: effectiveness and cost-effectiveness. In doing so, we ask (1) what is already known about the (cost-)effectiveness of DiPH interventions and (2) what factors should be considered when evaluating such interventions. Although the body of literature is growing rapidly, the existing evidence is limited and often poor. So far, the effectiveness of DiPH interventions in terms of user outcomes was mainly assessed short-term (i.e., pre- vs. post-intervention) and compared to no intervention conditions. When assessing cost-effectiveness, it is important to distinguish between the perspectives of different types of decision makers (individuals, companies, healthcare payer, other public payers). In general, the estimated return on investment from a company perspective is positive in studies with a low quality of evidence, but negative in randomized controlled trials (RCTs) with a high quality of evidence. We argue that the evaluation of DiPH interventions requires alternative evaluation methods that, unlike the traditional methods (e.g., RCTs) could generate evidence faster and detect evidence gaps to be addressed in future studies.