In modern biotechnology, enzymes or other valuable biomolecules from microbes are essential because of their applicability in challenging reactions and most often, they provide many benefits compared with conventional chemical production processes concerning enantiomeric purity, environmental sustainability, and saving of expenses. Escherichia coli is most often the host of choice when it comes to the construction of metagenomic libraries and to the expression of recombinant enzymes. This chapter discusses possible alternative host systems such as Pseudomonas putida. It reviews current achievements in the field of metagenomic gene and enzyme discovery and reviews advancing techniques. The chapter further presents a selection of different screening methods that were currently applied for the functional detection of diverse classes of biocatalysts, including sequence-based, function-based, and gene expression-based screening. The screenings and assays basically rely on color changes of substrates, visible substrate degradation, genetic complementation, and growth dependency or on morphology studies.