The Australian Energy Market Operator AEMO is responsible for operating Australia's largest gas and electricity markets and power systems. The national electricity market NEM commenced operation as wholesale spot market in December 1998. It interconnects five regional market jurisdictions Queensland, New South Wales including the Australian Capital Territory, Victoria, South Australia, and Tasmania. The National Electricity Market NEM operates on one of the world's longest interconnected power systems, stretching from Port Douglas in Queensland to Port Lincoln in South Australia and across the Bass Strait to Tasmania a distance of around 5,000 kilometers.This presentation will provide an overview of NEM. The presentation will focus on the electricity market dispatch and pricing. Types of ancillary services and its effect on electricity market will also be presented. Finally, the presentation will include future challenges for NEM.