The experience sampling method (ESM) is also referred to as the daily diary method or ecological momentary assessment (EMA). The method measures at the moment itself, instead of afterwards, and at several times a day in the daily life of an individual. The method can be implemented in various ways, for example by asking questions via pop-ups on smartphones or by using data from wearables, sensors or other digital biometers. In this way, the researcher can gain insight into the feelings, thoughts, behavior, environmental factors, heart rate, stress or sleep rhythm of individuals in daily life. The intensive longitudinal data collected with ESM provide new insights about mental health, for example about changes that occur within the individual and differences between persons. Knowing what works for one and not for the other and why, enables us to offer tailor-made interventions.