An avalanche photodiode circuit, comprising an avalanche photodiode typically connected in parallel to a capacitor, in which there is provided a current shunt circuit which activates to shunt current from the avalanche photodiode in response to detecting a decrease in the impedance of the avalanche photodiode, typically measured by determining the slope of the voltage across the avalanche photodiode. By using this circuit, the avalanche photodiode can be protected from sudden increases in incident light level decreasing the impedance of the avalanche photodiode to an extent that the energy such as is stored in the capacitor can damage the structure of the avalanche photodiode.