  Our recent progress on time-dependent modeling of the multiwavelength spectra
and variability of blazars with leptonic and hadronic jet models is reviewed.
Special emphasis is placed on X-ray spectral variability of low-frequency
peaked (LBLs) and intermediate BL Lac objects (IBLs). As an example, recent
observational and modeling results of an extensive multiwavelength campaign of
BL Lacertae in 2000 are presented. It is demonstrated how combined spectral and
variability modeling of LBLs and IBLs can significantly constrain emission
models and potential variability scenarios. In the case of BL Lacertae, the
variability appears to be driven primarily by fluctuations in the spectral
index of the non-thermal, ultrarelativistic electron population in the jet.
Such constraints allow us to refine predictions of the intrinsic GeV gamma-ray
emission and the dominant electron cooling mechanism in these objects.
