Biological tissue is a group of cells that perform a similar function.
The study of tissues is known as histology, or, in connection with disease, histopathology.
The classical tools for studying the tissues are the wax block, the tissue stain, and the optical microscope, though developments in electron microscopy, immunofluorescence, and frozen sections have all added to the sum of knowledge in the last couple of decades.
With these tools, the classical appearances of the tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.
There are four basic types of tissue in the human body. These compose all the organs, structures and other contents.
Examples of tissue in other multicellular organisms are vascular tissue in plants, such as xylem and phloem. Plant tissues are categorized into the parenchyma, the collenchyma, and the sclerenchyma.