Phosphorus (P) is an essential nutrient that is integral to lipids, nucleic acids and various metabolites, and also binds proteins covalently in ways that may alter their catalytic activities and interactions with other proteins. Phosphate (PO₄ ³⁻), both inorganic and organic, is the major source of P for nearly all microbes, algae and plants, although in some environments organic PO₄ ³⁻ molecules (including nucleic acids, phospholipids and phosphonates) comprise a significant proportion of the available P. Many natural environments have low levels of available P, which limits the growth of plants, algae and microbes. These organisms have developed a diversity of strategies to scavenge PO₄ ³⁻ from external sources, to recycle and balance P utilization within the cell in response to environmental conditions, and to coordinate cell growth and division with P availability. Furthermore, agricultural lands may be depleted of many nutrients including nitrogen (N) and P. Fertilizers with high N and P contents are liberally applied to many millions of acres of farmland globally, and these could be leached from the soil to contaminate lakes, rivers, ponds and coastal waters. Research groups are now starting to understand the complexity of P cycling in the environment and the molecular mechanisms associated with the acclimatisation of photoautotrophic and heterotrophic organisms to P limitation. In this chapter, P availability in the natural environment is discussed, and the physiological and molecular strategies used by algae (in the context of other organisms) for the efficient capture of external P, to recycle P-containing molecules in the cell, and to reconfigure cellular metabolism to sustain viability in a P-limited landscape, are emphasised.