Commercial production of carotenoids from microorganisms competes mainly with synthetic manufacture by chemical procedures. Efficient stimulation of carotenoid biosynthesis is expected to promote accumulation of carotenoid by microbes. This review describes the variety of environmental and cultural stimulants studied during the last few decades which enhance volumetric production and cellular accumulation of commercially important carotenoids from microalgae, fungi and bacteria. Stimulation of carotenoid production by white-light illumination and temperature fluctuation is discussed along with supplementation of metal ions, salts, organic solvents, preformed precursors and several other chemicals in the culture broth. Reports on the improvements in yield are reviewed and assessed from a biotechnology point of view.