This overview provides guidelines for the characterization of recombinantly expressed proteins (e.g., verifying primary structure and appropriate post-translational modifications), along with methodologies for characterizing the proteins according to size, X-ray structure, absorbance, biological activity, and subunit structure. A flow chart presents a suggested path for fully characterizing recombinant protein and involves equipment for HPLC, mass spectrometry, circular dichroism, NMR and fluorescence spectroscopy. Also covered are sources and consequences of contamination in protein solutions.