A great deal of interest is currently being shown in filter structures with varying degrees of wavelength diversity or an ability to control the intensity of transmitted or reflected light. Potential applications cover a wide spectrum, from areas such as image processing and optical computing to the commercial world of smart windows and automatically dimming rear-view mirrors on vehicles. Most of the device designs exploit materials which respond to external stimulus via well-known optical effects including: Thermochromic processes (eg as in phase transition materials) Electrochromic processes (eg as a result of ion insertion) Thermo-optic processes (exploiting the temperature dependant of refractive index) Electro-optic effects (exploiting field dependant refractive index) Non-linear effects (intensity dependant refractive index) Many of the effects observed are fairly small and it is generally important to enhance the tuning range or degree of contrast change through the exploitation of multilayer coating designs. Some active devices also exploit the use of polarised light and the magnitude of optical phase change produced in birefringent media. Examples are the groups of liquid crystal materials, which can realise large changes in optical anisotropy as a result of electric field-induced molecular ordering processes. This paper presents a review of some of the technologies available for active filter application, highlighting examples of devices produced and the potential offered by emerging materials in thin film form.