  We present the results of variability power spectral density (PSD) analysis
using multiwavelength radio to GeV\,$\gamma$-ray light curves covering
decades/years to days/minutes timescales for the blazars 3C 279 and PKS
1510-089. The PSDs are modeled as single power-laws, and the best-fit spectral
shape is derived using the `power spectral response' method. With more than ten
years of data obtained with weekly/daily sampling intervals, most of the PSDs
cover ~2-4 decades in temporal frequency; moreover, in the optical band, the
PSDs cover ~6 decades for 3C 279 due to the availability of intranight light
curves. Our main results are the following: (1) on timescales ranging from
decades to days, the synchrotron and the inverse Compton spectral components,
in general, exhibit red-noise (slope ~2) and flicker-noise (slope ~1) type
variability, respectively; (2) the slopes of $\gamma$-ray variability PSDs
obtained using a 3-hr integration bin and a 3-weeks total duration exhibit a
range between ~1.4 and ~2.0 (mean slope = 1.60$\pm$0.70), consistent within
errors with the slope on longer timescales; (3) comparisons of fractional
variability indicate more power on timescales $\leq$100\, days at $\gamma$-ray
frequencies as compared to longer wavelengths, in general (except between
$\gamma$-ray and optical frequencies for PKS 1510$-$089); (4) the normalization
of intranight optical PSDs for 3C\,279 appears to be a simple extrapolation
from longer timescales, indicating a continuous (single) process driving the
variability at optical wavelengths; (5) the emission at optical/infrared
wavelengths may involve a combination of disk and jet processes for
PKS\,1510$-$089.
