Abstract Under steady laser illumination, aperiodic spatio-temporal oscillations are observed in the light transmitted and reflected by parallel-sided samples of the ferroelectrics PMN (lead magnesium niobate, Pb(Mg 1/3 Nb 2/3 )O 3 ), PLZT (lanthanum doped lead zirconium titanate, Pb(Zr 1– x Ti x )O 3 :La), BNN (barium sodium niobate, Ba 2 NaNb 5 O 15 ) and Ce:SBN (cerium doped strontium barium niobate, Ce 3+ :Sr x Ba 1– x Nb 2 O 6 ). There are two distinct mechanisms for such oscillations, namely (1) unistable thermal focusing oscillations, which occur in BNN and Ce:SBN and (2) bistable or multistable Fabry-Perot switching oscillations, as observed in PMN and PLZT. The two types of oscillation have characteristic time scales differing by several orders of magnitude. We derive temperature profiles as a function of incident laser power, beamwidth and sample thickness, which quantitatively predict the spatio-temporal characteristics of the transmitted beam patterns.