Many protists have protective shells or tests, usually made from silica (glass) or calcium carbonate (chalk). Protists are a diverse group of eukaryote organisms that are not plants, animals, or fungi. They are typically microscopic unicellular organisms that live in water or moist environments.

Protists shells are often tough, mineralised forms that resist degradation, and can survive the death of the protist as a microfossil. Although protists are typically very small, they are ubiquitous. Their numbers are such that their shells play a huge part in the formation of ocean sediments and in the global cycling of elements and nutrients.

The role of protist shells depends on the type of protist. Protists such as diatoms and radiolaria have intricate, glass-like shells made of silica that are hard and protective, and serve as a barrier to prevent water loss. The shells have small pores that allow for gas exchange and nutrient uptake. Coccolithophores and foraminifera also have hard protective shells, but the shells are made of calcium carbonate. These shells can help with buoyancy, allowing the organisms to float in the water column and move around more easily.

In addition to protection and support, protist shells also serve scientists as a means of identification. By examining the characteristics of the shells, different species of protists can be identified and their ecology and evolution can be studied.

Cellular life likely originated as single-celled prokaryotes (including modern bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes include organisms such as plants, animals, fungi and "protists". Protists are usually single-celled and microscopic. They can be heterotrophic, meaning they obtain nutrients by consuming other organisms, or autotrophic, meaning they produce their own food through photosynthesis or chemosynthesis, or mixotrophic, meaning they produce their own food through a mixture of those methods.

The term protist came into use historically to refer to a group of biologically similar organisms; however, modern research has shown it to be a paraphyletic group that does not contain all descendants of a common ancestor. As such it does not constitute a clade and is not currently in formal scientific use. Nonetheless, the term continues to be used informally to refer to those eukaryotes that cannot be classified as plants, fungi or animals.

Most protists are too small to be seen with the naked eye. They are highly diverse organisms currently organised into 18 phyla, but are not easy to classify. Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting large numbers of eukaryotic microbial communities have yet to be discovered. As eukaryotes, protists possess within their cell at least one nucleus, as well as organelles such as mitochondria and Golgi bodies. Many protists are asexual but can reproduce rapidly through mitosis or by fragmentation; others (including foraminifera) may reproduce either sexually or asexually.

In contrast to the cells of bacteria and archaea, the cells of protists and other eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic, but there are exceptions, and some marine protists are neither single-celled nor microscopic, such as seaweed.