Recent multi-wavelength surveys of a few AGN have given us the opportunity to constrain well the cross-correlation between the X-rays and the UV/optical variations in these objects. The variations in the UV lead the variations detected at longer wavelengths in almost all cases where good quality light curves, in many wavebands, exist. However, there have been indications that the optical variations lead the UV variations on the longest sampled time scales in a few objects. This could indicate the presence of accretion rate variations which propagate inwards. We use well sampled, long light curves of a few Seyfert galaxies to compute the time lags on both long and short time scales. We will present the results regarding the dependence of the observed time-lags on the probed time-scale, and we will discuss possible constraints on various models for the observed optical/UV variations in AGN.