Digital technologies, such as mobile devices and wearable sensors, are ingrained in daily life, making them a promising vehicle for delivering health behavior interventions. However, a critical challenge that undermines the utility of digital interventions is the suboptimal engagement of participants, where participant engagement is defined as the investment of physical, cognitive, and affective energies in a focal stimulus or task. Recent years have seen substantial growth in research aiming to understand how to increase engagement with digital interventions. This paper highlights several limitations of the existing evidence that restrict its scientific and practical utility and discusses opportunities for advancing the science of engagement with digital interventions. Synthesizing the current body of evidence, we call for conceptualizing digital interventions as a collection of stimuli (e.g., notifications, reminders) and tasks (e.g., open the mobile app, practice a relaxation technique) and considering engagement with digital interventions as a process rather than a state (i.e., momentary conditions/experiences) or trait (i.e., a relatively stable disposition). This approach has the potential to enhance scientific rigor and transparency in measuring, reporting, and interpreting engagement with digital interventions that would ultimately serve to bolster progress towards developing strategies for optimizing engagement.