Photosynthetic microalgae continue to be of great interest as a promising class of feedstock for biofuels and higher-value chemicals and oils, with the potential for contributing to liquid transportation fuel supplies while reducing GHG emissions through the displacement of fossil petroleum-based fuel and chemical product usage.� However, to significantly contribute to fuel supplies will require algae cultivation scale-up to large aggregated quantities of biomass and fuel feedstock production that will necessarily impose huge demands for land, water, supplemental CO2, and other key macronutrients such as nitrogen and phosphorus. �The resulting resource requirements will impose constraints on the level of production that can be sustainably supported. �This presentation provides a high-level analysis and review of the current status and future prospects for these key resource demand and constraint challenges for the scale-up of autotrophic microalgal biofuel production. �The analysis is focused on resource demand issues and assumes that the current techno-economic challenges facing the affordable scale-up of algal biofuels production can be overcome.� Emphasis is placed on the USA, although the issues are generally relevant globally.� Developing microalgae production approaches that can effectively use non-fresh water resources, capture and recycle both water and macronutrients from downstream algae harvesting and processing, and minimize requirements for both fresh make-up water and commercial nutrients will help reduce, but not eliminate, resource constraints.� Providing supplemental CO2 resources for enhanced and affordable algae production may be the biggest challenge.� Based on current understanding, resource demand could constrain U.S. algal biofuel production to levels below 10 BGY.