Hydrophobic interaction chromatography (HIC) is one of the basic purification procedures in the biosciences. However, because of its complexity, it has not gained the same foothold in the methodological repertoire of protein chemistry as has affinity chromatography or ion exchange chromatography. This is mainly a result of the lack of a general optimization procedure for the reversible adsorption and elution of a novel protein to be purified. Further problems arise from the fact that most commercial hydrophobic adsorbents are inadequate for an ideal performance in downstream processing procedures, because these media are too hydrophobic and elution of proteins in their native state is often impossible. Therefore, as in the 1970s, a bioscientist of today has to be capable of synthesizing a small library of hydrophobic gels from which he or she can then select and optimize the ideal matrix for their special needs. In addition, a general optimization method employing the critical hydrophobicity concept has now been devised that should allow the application of HIC methodology to many hitherto unpurified proteins. In this chapter, the reader is first introduced to the theoretical background (multivalence, negative cooperativity, adsorption hysteresis) of the binding of protein ligands to hydrophobic supports, so that they will be capable of independently adapting HIC to a novel protein. Then a simple nontoxic method is described for the synthesis of HIC-gel libraries consisting of a homologous series of uncharged alkyl-Sepharoses of three chain lengths (butyl, pentyl, and hexyl Sepharose) prepared with different degrees of separation. From this series a critical hydrophobicity gel can then be selected and employed for critical hydrophobicity HIC. A detailed example for the chromatography of human fibrinogen is given that has been employed as a one-step procedure for the purification of fibrinogen from human plasma.