While a large surface area combined with short diffusion distances make fish gills well suited for gas exchange, these properties leads to costly water and ion fluxes and exposure to toxic substances and pathogens. Thus, gill morphology is likely to be a compromise between opposing demands. It has become clear that some fishes have the ability to modify gill structure in response to environmental parameters such as oxygen levels and temperature. Maybe the most dramatic example of gill plasticity is the adaptive and reversible changes in gill surface area displayed by crucian carp (Carassius carassius) and goldfish (Carassius auratus). Here, a cell mass is filling up the space between the lamellae during normoxic and cold conditions (i.e. when oxygen demands are low). Hypoxia or high temperature induce apoptosis and suppress mitosis in the interlamellar cell mass causing it to retract and the lamellae to protrude. The functional importance of oxygen and temperature induced changes in gill morphology and the underlying mechanisms are discussed.