There are several key challenges in evaluating eHealth interventions and applications. One major challenge is attrition, as adherence tends to be low for online interventions. High attrition needs to be accounted for using appropriate statistical methods and reporting "attrition curves" over time. Another challenge is controlling the control group, as similar online interventions may be accessible to both groups. Maintaining participant identity and preventing re-registration can also be issues. Other difficulties include intervention complexity, data quality, messages being identified as spam, and cybersquatting. Addressing these methodological challenges is important for rigorously evaluating eHealth technologies.