The efficacy of ophthalmic semi-solid hydrogels is mostly based on an increase of ocular residence time, via enhanced viscosity and mucoadhesive properties. Preformed and in particular in situ gelling systems improve bioavailability and decrease the side effects induced by the systemic absorption of topically applied ophthalmic drugs. Since increased viscosity often causes the discomfort of blurred vision and foreign body sensation, it is important to assess the optimal range of viscosity as well as the most suitable rheological behavior which will ensure good efficacy and tolerance.