We have infiltrated photonic crystal fibers with liquid crystals, thus creating, to our knowledge for the first time, what we call Liquid Crystal Photonic BandGap (LCPBG) fibers. We have demonstrated thermal, electrical and all-optical tunability of the optical transmission properties of these fibers, thus providing broadband spectral switching and filtering functionalities which may be operated linearly as well as nonlinearly. For instance, in a continuous thermo-optic mode, the photonic bandgaps of a LCPBG fiber can be tuned with a sensitivity of a few nm/°C. In a thermo-optic threshold mode, a temperature change as small as 0.4°C can cause optical switching with an extinction ratio as high as 80 dB in the near-infrared. In another nonlinear mode, all-optical modulation up to 2 kHz frequency of an infrared probe signal has been achieved by using a pulsed, 532 nm pump source of a few milliwatts optical power. In addition, since the mode-field diameter of the LCPBG fiber is similar to that of standard, single-mode fibers, these devices can be inserted into existing telecom transmission links without significant additional losses. This paper gives a basic overview of the optics and the materials involved in LCPBG fibers, in particular "tuning in" on how the optical transmission properties of these fibers depend on various liquid crystal aspects, such as mesomorphism, optical and electrical parameters, and the alignment of liquid crystals in cylindrical geometry.