The field of directed evolution of oxygenases (mono-, di- and epoxygenases) is rapidly advancing as an increasing number of success stories indicate. A significant number of screening systems have been developed to specifically improve oxygenase properties. Oxygenases will become very valuable biocatalysts for synthetic applications in industry when stability, cofactor and activity properties match industrial demands. This review summarizes screening systems and principles of screening systems that have been used for directed evolution of oxygenases. Sections on mutagenic conditions, mutant library size and property improvements provide a comprehensive picture on performance and limitations of current directed evolution methodologies for oxygenases. A discussion of challenges in the directed evolution of oxygenases for industrial exploitation concludes this review.