Foraminifera (nickname forams) are small, one-celled organisms that typically have shells. Most forams make their shells from calcium carbonate, but some use other materials (aragonite or silica), and chambered structure, and is used to determine species. Although many forams are microscopic, some are as large as 20 cm (almost 8 inches), and still just made of one cell. Forams stretch out parts of their cell in skinny projections (pseudopodia) that can be many times as long as they are wide. Pseudopodia are used in various ways to accomplish most things forams need to do: move around, absorb oxygen, take in nutrients, send waste out, carry around photosynthetic algae, or even create nets to catch prey. Forams are so abundant that they can be found in a scoop of sediment from the ocean bottom nearly everywhere in the world.

The shells of tiny foraminifera (nickname forams) have left a fossil record dating to the beginning of the Cambrian, about 550 million years ago. The earliest forams, the benthic forams, lived in ocean bottom sediments. Later, during the Jurassic, floating forams evolved (planktic forams). Both benthic and planktic forams underwent many periods of diversification and extinction that show up as changes in the distribution of fossil foram shells in sediments of different ages. Species of forams have tended to persist on Earth for about 0.5-20 million years, a short period relative to geologic time. Scientists can, therefore, determine the age of sediments based on the fossil foram shells found in them. Because some forams are very sensitive to environmental conditions such as temperature, oxygen concentration, salinity, and food availability, their fossils offer evidence of what conditions were like in the ancient past. Changes in foram distributions over time track environmental changes, including global climate change.