Matching the broad-band emission of active galaxies with the predictions of theoretical models can be used to derive constraints on the properties of the emitting region and to probe the physical processes involved. AP Librae is the third low frequency peaked BL Lac (LBL) detected at very high energy (VHE, E>100GeV) by an Atmospheric Cherenkov Telescope; most VHE BL Lacs (34 out of 39) belong to the high-frequency and intermediate-frequency BL Lac classes (HBL and IBL). LBL objects tend to have a higher luminosity with lower peak frequencies than HBLs or IBLs. The characterization of their time-averaged spectral energy distribution is challenging for emission models such as synchrotron self-Compton (SSC) models.

The status of Very High Energy (VHE, E$\,{>}\,$50 GeV) gamma-ray observations of BL Lacertae objects is presented. The catalogue of well-established BL Lacertae objects detected at VHE energies contains seven members, and there have been recent reports of the detection of another four. All are nearby, X-ray bright sources. The temporal, spectral and broadband multi-wavelength properties of the sources are reviewed and possible implications for the gamma-ray production mechanism discussed. The most recent detections provide more stringent constraints on the cosmic extragalactic background light level and imply that the Universe is more transparent to VHE gamma radiation than previously thought.