Chronic diarrhea is defined by a decrease in stool consistency and a bowel frequency of more than 3 times per day, lasting for at least 4 weeks. Multiple underlying causes may be responsible for chronic diarrhea. There are four main pathomechanisms for chronic diarrhea: osmotic diarrhea, secretory diarrhea, infectious diarrhea and bowel dysmotility. Overlaps between these mechanisms may exist. A stool collection over a 72-hour period frequently allows to classify diarrhea into one of these four entities. Such classification finally helps for the identification of underlying cause(s), thereby allowing rational diagnostic measures. It also limits the costs of diagnostic workup. This article aims to present the main causes of chronic diarrhea, the diagnostic steps to perform and to provide a guideline for clinicians in daily practice.