Filtration is largely an utilitarian exercise designed to attain a practical end; in this instance, organism removal. Its pragmatic goals require only a 'when and how' type of knowledge. This book is intended for those who seek a deeper understanding of the 'whys' of filtration in the hope of abetting the 'hows'. Filtration takes place within a matrix of seemingly disparate technical operations. They may be judged to be unconnected and independent, but possibly only because their influences on the filtration process are not immediately apparent. Nevertheless, they constitute the envelope of pharmaceutical practices within which filtrations have their setting. What is sought in this book is an approach to an understanding of this universe. This approach seeks to emphasize that filtrations are not practiced through a selected assemblage of independent parameters. Rather they are largely a resultant of interactions and relationships among the several operational choices that yield a balanced outcome. The removal of organisms from drug preparations finds its ultimate expression in the production of sterile products. Thus, sterilizing filtrations inevitably beg definition of the 'sterilizing' filter. An elaboration of the present view of the 'sterilizing filter' is undertaken. Previously, the identity of the sterilizing filter could be known with certainty. It required only the confirmation of a filter integrity test, which correlated the filter's pore size rating with an assured and necessary level of organism retention. This vouched for a sterile effluent, assuming proper operations. It is now understood that whether a filter can deliver a sterile preparation depends upon a proper conjunction of several factors; a positive integrity test is but one of them. This book examines a construct of: pore size; organism and organism size; suspending fluid; the polymer matrix; and operational conditions. Achieving the correct integration of these factors requires documented experimental confirmation. Identifying a particular filter and affirming its organism retention qualities in a singular filtration no longer suffices. In the process, it will be found that there are connections, some perhaps unexpected, among different aspects of the filtration picture; as between the compressibility of gases, and the design of filter housings; and as between the acceptance of the economic burden of slower productivity as the price of effluent quality. In this view, the mechanisms of particle retention have relevance; the integrity test of the filter, when, where and how has importance; the nature of adsorptive bonding has meaning; and disciplines such as the technology of membrane casting have significant consequences. The topics selected for fuller discussions of their organism retention influences are: the nature of membrane porosity; the matching of pore and organism sizes; the fluid's compatibility with both pores and organisms; the adsorptive effects of the filter's polymeric composition; the influence of the operational conditions; the phenomenon of grow-through;, and a consideration of the several mechanisms of organism retention. Ancillary aspects of related filtration practices, filter design, integrity testing and applications are included, as is also a relevant bibliography of some 260 references. The book is a compilation of many years of observations within the filtration processes, but also hundreds of publications.