Peroxisomes are round in shape, and a cell contains several hundred of them. The peroxisome is contained in a membrane of proteins and lipids, and the inside contains oxidation enzymes. It is thought that that peroxisomes developed due to the increase of oxygen in the environment. The three oxidation enzymes contained in the peroxisome are D-amino acid oxidase, urate oxidase, and catalase. Catalase, which is the enzyme that specifically decomposes hydrogen peroxide, is contained in all peroxisomes. Like mitochondrions, peroxisomes are self-replicating. Though peroxisomes resemble lysosomes, they have a crystalline structure inside a sac and are not formed in the Golgi apparatus. Peroxisomes are made as a phospholipid bilayer.

It was formerly believed that peroxisomes were formed by the smooth Endoplasmic Reticulum (ER). It is now known that peroxisomes are self-assembling, since their "life-span" is only a day. The process of reproduction is called peroxisomal biogenesis. The proteins that are found in the membrane of the peroxisome are made in the ribosome. These proteins have genetic information brought by RNA to the ribosome, since peroxisomes do not contain their own DNA. Peroxisomes have membrane proteins to import these proteins from the cytosol into their interiors, because they have no genome or ribosomes. This process happens by means of peroxisomal targeting signals (PTSs), which are a sequence of amino acids synthesized in the ribosome.

The main function of peroxisomes is to isolate atoms with hydrogen by adding oxygen, making hydrogen peroxide, or to break down hydrogen peroxide by adding oxygen to make water and oxygen gas. Lysosomes are responsible for protection of a cell from the hydrogen peroxide that it makes, and use the enzymes in peroxisomes to oxidate. In the leaves of plants, they oxidize a side chain product of the reaction that makes CO2. This reaction is called photorespiration. Another function of peroxisomes is converting fatty acids to sugars, which is utilized by germinating seeds as a source of energy before they can photosynthisize. This process is called the glyoxylate cycle where glucose is oxidized by glycolysis to produce energy. Peroxisomes also decompose entirely some fatty acids.