Utilizing a focused laser beam manipulated through computer-controlled mirrors, and capable of "writing" spatiotemporal temperature fields on a surface, we explore here the fundamental impact of localized spatiotemporal perturbations on a simple reaction-diffusion system. Our two-dimensional model system is the low-pressure catalytic oxidation of CO on Pt(I 10), a reaction exhibiting well-understood spatiotemporal patterns. In the simplest case the laser spot causes the ignition of a reaction wave by a single critical "kick" at a selected surface location. The cooperativeness between two local subcritical perturbations separated in time and/or space is then explored. A temperature heterogeneity moving along a line may ignite waves along its path, or can drag preexisting pulses. In the oscillatory region we find localized beat patterns when the laser spot moves along a circle. The ratio between the underlying natural oscillation frequency and the forcing (circle-writing) frequency is important here. Finally we demonstrate how pulses, the basic building blocks of chemical patterns, can be modified, guided, and erased and how the overall reaction rate can be increased through localized actuation. Computational studies supplement and rationalize the experimental findings. (C) 2003 Elsevier Science (USA). All fights reserved.