An LED lighting system is disclosed comprising: a heatsink; a plurality of strings of LEDs each string comprising one or more LEDs each having a junction and being mounted on the heatsink, and a controller comprising a memory unit and a processor and being configured to supply a current to each of the strings of LEDs; the processor comprises: a first temperature estimation subunit configured to generate an estimate of a temperature of the junction of a one of strings of LEDs; a heatsink temperature estimation subunit configured to estimate a temperature of the heatsink unit from the first estimate; and a second temperature estimation subunit configured to provide an estimate of a temperature of the junction of a second string of LEDs, from the estimated temperature of the heatsink. A controller for and method of operating a plurality of LEDs are also disclosed.