Variability in active galactic nuclei (AGN) spans a wide range of timescales from a few 100 s (gamma rays) to a few years (optical and radio). Here, we present recent models and constraints on the AGN jet properties and black hole mass. We first study the effects of radiation pressure and drag which stabilize the outflow with Lorentz factors of 1.1 - 7, used as inputs to a model of jet based variability. Then, we describe the radio core shift effect in blazar 3C 454.3 using the 4.8 GHz - 36.8 GHz long term radio light curves. We infer a magnetic field strength of 0.07 - 1.43 G at 1 pc and 0.03 - 0.17 G at the core. A Fourier periodogram analysis yields power law slopes ranging between -1.6 and -3.5, and gives bend timescales ranging between 0.52 and 0.66 yr. Flares originate from multiple shocks in a region at a distance of a few to tens of parsecs from the central region. We then study the ~ 1.6 yr Kepler light curve of the blazar W2R 1926+42. A bend timescale of 3.1 - 12.6 hours is inferred. Using this and the normalized excess variance, we infer a black hole mass of (1.5 - 5.9) x 10^7 M_sun. We argue that the observed variability and an inferred weak and short duration quasi periodic oscillation could be due to jet based orbital features and others such as shocks and turbulence.