In vitro (Latin: in glass) studies in experimental biology are those that are conducted using components of an organism that have been isolated from their usual biological surroundings in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments are commonly called "test tube experiments". In contrast, in vivo work is that which is conducted with living organisms in their normal, intact state, while ex vivo studies are conducted on functional organs that have been removed from the intact organism.

Common examples of in vitro experiments include work that uses (a) cells derived from multicellular organisms (cell culture or tissue culture), (b) subcellular components (e.g.mitochondria or ribosomes), (c) cellular or subcellular extracts (e.g. wheat germ or reticulocyte extracts), or (d) purified molecules in the test tube (often proteins, DNA, or RNA, either individually or in combination).

In vivo (Latin for "within the living") is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro ("within the glass", i.e., in a test tube or petri dish) controlled environment. Animal testing and clinical trials are two forms of in vivoresearch. In vivo testing is often employed over in vitro because it is better suited for observing the overall effects of an experiment on a living subject. The maxim in vivo veritas ("in a living thing [there is] truth")^[1] used to describe this type of testing is a play on words fromin vino veritas, in wine [there is] truth.

In microbiology in vivo is often used to refer to experimentation done in live isolated cells rather than in a whole organism, for example, cultured cells derived from biopsies. In this situation, the more specific term is ex vivo. Once cells are disrupted and individual parts are tested or analyzed, this is known as in vitro.

Ex vivo (Latin: "out of the living") means that which takes place outside an organism. In science, ex vivo refers to experimentation or measurements done in or on tissue in an artificial environment outside the organism with the minimum alteration of natural conditions. Ex vivo conditions allow experimentation under more controlled conditions than is possible in in vivo experiments (in the intact organism), at the expense of altering the "natural" environment.

The term ex vivo is often differentiated from the term in vitro ("within the glass") in that the tissue or cells need not be in culture; these two terms are not synonymous.

In cell biology, ex vivo procedures often involve living cells or tissues taken from an organism and cultured in a laboratory apparatus, usually under sterile conditions with no alterations for up to 24 hours. Experiments lasting longer than this using living cells or tissue are typically considered to be in vitro. One widely performed ex vivo study is the chickchorioallantoic membrane (CAM) assay. In this assay, angiogenesis is promoted on the CAM membrane of a chicken embryo outside the organism (chicken).