Insomnia, defined by difficulty falling asleep or remaining asleep, early morning awakening and/or non-restorative sleep, and daytime consequences, is an important public health issue with a significant negative impact on individuals' physical and social performance, ability to work and quality of life, as well as on society as a whole. Chronic insomnia warrants treatment in the majority of cases, but it is often under-treated. Primary insomnia occurs independently of other factors, and is possibly related to a general psychophysiologic hyperarousal. Other types of insomnia occur in association with various conditions such as psychiatric disorders, medical disorders (e.g., chronic pain, dysfunction and movement disorders), circadian rhythm disorders and medication or substance use. These types of insomnia are diagnosed more frequently in the clinic. As a result, insomnia is traditionally viewed and treated as a symptom rather than a disease, with the majority of therapies aimed at resolving underlying medical factors. However, it is important to clearly establish whether co-morbidities are causative for or simply co-exist with insomnia, in order to recommend the most appropriate treatment and optimize treatment outcomes. Difficulties still arise when categorizing insomnia subtypes. Here, we highlight some of the major challenges for future research in classifying both primary insomnia and insomnia related to or associated with various conditions, and their relevance to primary care.