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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 xylemIn vascular plants, xylem is the tissue that carries water up the root and stem. In trees, it constitutes wood — hence the word is derived from Greek ξύλον xúlon "wood, timber". Together with phloem, xylem is one and phloemIn vascular plants, phloem is the tissue that carries organic nutrients, particularly sucrose. In trees, the phloem is part of the bark, hence the name, derived from the Greek word for "bark". See also xylem. Phloem sap moves from sugar source''s to sugar. Plant tissues are categorized into the parenchymaThe parenchyma is the functional parts of an organ in the body (i. the nephrons of the kidney, the alveoli of the lungs). In plants, parenchyma are unspecialized, thin-walled cells that make up the bulk of most nonwoody structures. Parenchyma cells in bet, the collenchymaCollenchyma plant tissue is composed of elongated cells with unevenly thickened walls. They provide structural support, particularly in growing shoots and leaves. Collenchyma tissue composes, for example, the resilient strands in stalks of celery. Its gro, and the sclerenchymaSclerenchyma is a tissue found in vascular plants. It is composed of tough fibers and thick-walled sclereids both of which contain lignin. The cells are dead at maturity, and they are packed tightly together. Sclerenchyma serves to support and protect the.