Optical filters play important roles in photonic processing systems and optical communications as well as in many optical systems. Even more important and critical in the use of these filters are the types, the bandwidth, and center wavelength which can be tuned to desired regions and passband. In this chapter, a digital filter design method is employed to systematically design tunable optical filters with variable passband and center wavelength=frequency characteristics for lowpass, highpass, bandpass, and bandstop types. Potential applications of such filters and the existing design techniques in wavelength-division multiplexing (WDM) and dispersion equalization are also discussed in this chapter. Basic optical filter structures, namely, the first-order all-pole optical filter (FOAPOF) and the first-order allzero optical filter (FOAZOF) are described, hence the design process of tunable optical filters and the design of the second-order Butterworth lowpass, highpass, bandpass, and bandstop tunable optical filters are described. An experimental development of the first-order Butterworth lowpass and highpass tunable fiber-optic filters is given.