Contact lens use has increased as advances have improved the performance of the lenses as well as their cleaning and disinfecting solutions. Contact lenses can be used as an alternative to glasses, as treatment for various eye conditions, as well as a tool for diagnosis of specific eye problems. The increased awareness of availability of the lenses has lead to common misconceptions. Contact lenses are not a care-free replacement for glasses; similarly, contact lens use is not universally applicable to all eye conditions. Appropriate dispensing, care for lenses, routine follow-up with an eye care professional, and timely replacement are all important components of safe contact lens use.

Most frequently, contact lenses are employed to correct refractive error, or to "correct the need for glasses". The eye needs an additional lens to help it do its work when its optical system does not adequately focus on the retina (in the back of the eye). The "near-sighted" or myopic eye has an optical system which is too strong: the image is focused in front of the retina. The "far-sighted" eye is too weak: the image is focused behind the retina. The eye with astigmatism focuses a warped image on the retina. Lastly, the presbyopic or "aging" eye does not focus at reading distances. All of these different refractive errors cause a blurred image.

Different types of contact lenses are used for different situations. Broad categories of lenses include soft contact lenses and hard contact lenses. Both types of lenses have benefits and drawbacks. Differences include comfort, optical quality, wearing schedule and clinical application. Lenses should be prescribed by the trained eye-care specialist who is familiar with their different characteristics.

Routine soft contact lenses can be used to help the near sighted, the far sighted, the presbyopic, and, to some extent, the astigmatic eye. The lenses are very comfortable because they are soft and are usually well tolerated, even by the new contact lens wearer. The lenses are safe, if used properly, and are available for annual, quarterly, weekly, or even daily replacement. It is imperative, however, that proper cleaning and disinfecting methods are used and that the recommended wearing schedule is not exceeded Failure to do so may result in vision threatening eye problems such as infection or even loss of the eye. Eyes with large astigmatism may require special soft toric lenses. These lenses are more customized to correct the "warped" image and are therefore usually more expensive. The toric lens is stronger in a specific direction (i.e. vertical, horizontal, diagonal). It therefore has an axis or "direction" that must be specified by the eye-care specialist, and generally requires trial and error fitting.

Hard lenses are also available in a variety of styles. Theses lenses are generally considered to require an adaptation period because they are hard and initially less comfortable. Hard lenses are used for the same purposes as soft lenses but are especially suited to correction of high astigmatism and irregular corneas. The lenses can produce clearer images than soft lenses in these patients but must, in general, be removed and cleaned daily. To date, most evidence demonstrates that wearing lenses for long periods (i.e. for many days) greatly increases the risk of developing contact-lens related problems. Hard lenses are less prone to these problems All of these factors need to be taken into account by the patient and the eye care specialist when selecting the lenses.

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