Optical fiber communication involves the transmission through a fiber of an optical signal consisting of a carrier with superimposed information. The bandwidth of the optical signal is only a small fraction of the wavelength region that an optical fiber is capable of transmitting. The volume of information can therefore be increased by adding further simultaneously transmitted carriers of wavelengths sufficiently separated to avoid overlapping bandwidths - a process known as wavelength division multiplexing. The available wavelength region for each individual carrier is known as channel and the ability to combine and separate the signals carried by the various channels is an important system feature. Drop, add, multiplexing and demultiplexing are terms used to describe such operations, and narrow-band thin-film filters arranged to transmit one channel and reflect all others are ideal beam splitters for these applications. Thin-film narrow-band filters have a long history and are well understood. The basic principles involved in narrow-band filters for wavelength division multiplexing are, therefore, not new. What is new is the unprecedentedly high level of performance demanded by the application. Meeting these demands has required entirely new approaches to design, manufacture and testing of the components, currently operating in the near infrared around 1550nm. Even the terminology, such as insertion loss and crosstalk, breaks with tradition. Center-wavelength precision, peak transmittance and residual reflectance levels, uniformity, and environmental stability all present special challenges. In spite of all this such filters are being successfully designed and manufactured. This paper reviews the fundamental problems.