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.

• Epithelium - Lines, covers, protects, absorbs and secretes.
• Connective tissue - As the name suggests, connective tissue holds everything together. Blood is considered a connective tissue.
• Muscle tissue - Muscle cells contain contractile filaments that move past each other and change the size of the cell.
• Nervous tissue - cells forming the brain, spinal cord and peripheral nervous system

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.