Nasal hyperreactivity is an important feature of allergic and non-allergic rhinitis. This paper reviews the possible mechanisms behind hyperreactivity. Distinct mechanisms may play a role in allergic rhinitis--an inflammatory disease--and non-allergic rhinitis, mainly a non-inflammatory disease. In allergic rhinitis, particularly in perennial allergic rhinitis, there is a close connection between allergic response and non-specific hyperreactivity. In non-allergic rhinitis, a pathological entity comprising a heterogeneous series of diseases, understanding and measuring nasal hyperreactivity is much more difficult. A variety of methods to assess nasal hyperreactivity are available. Given the heterogeneity of mechanisms, the various patients groups and the lack of standardization in tests, it is not surprising that measurement of nasal hyperreactivity is not included in the diagnostic arsenal of the clinician.