Biodiesel has excellent fuel properties including greatly reduced life cycle greenhouse gas emissions relative to petroleum diesel.  Unfortunately, the availability of seed and waste oils is insufficient to make a truly significant impact on the diesel market.  Microalgae have the potential to directly convert sunlight and CO₂ into lipids that can be transesterified to biodiesel.  Intensive cultivation of these organisms can be done on under-utilized land using saline water resources and flue gas from power plants and other industrial sources.  Per-acre productivities have the potential to exceed oilseed crops by more than an order of magnitude.  Microalgae would represent a new feedstock for biofuels, one that does not compete with current food/feed/fuel markets.  The Department of Energy supported the Aquatic Species Program at the National Renewable Energy Laboratory from 1978-96 to collect and engineer microalgal strains for increased oil production.  In recent years, there has been a renewal of interest in this field, particularly in the private sector.  However, serious technological and economic hurdles remain to be addressed.  This talk will be a high-level overview of past research, technology paths leading forward, and an evaluation of the overall potential for microalgae to become a significant energy resource in the future.