In recent years, the role of the microbial ecosystem in both human and animal health has become more prominent (Finegold 2008; Ley et al. 2006; Murphy 2004; Turnbaugh et al. 2009; Turnbaugh et al. 2006; Xu and Gordon 2003). The “microbial organ” is at last getting its due as a playing a part in health as well as production parameters (Lyte 2010). Though much of this research has focused on the effects of the microbial communities and cross-ommunication with the host in and on humans, increasing amounts of research has delved into the microbial organ of animals (Freestone and Lyte 2010). A new hypothesis has recently been advanced by Dr. Mark Lyte that probiotics may function as a drug as a delivery mechanism for neuroactive and bioactive compounds that affect the host. 
 In light of these changes in our understanding of intestinal microbial ecology based on new molecular and older culture-based methods, a revised vision of the role of Direct fed Microbials and prebiotics in animal agriculture was necessary. With this in mind, an American Dairy Science Association DISCOVER conference was held in 2009 on “Probiotics in Animal Agriculture: Science and Mechanisms of Action”. Following discussions with Dr. Gilliland and others at that conference, it was decided that a “state of the art” book needed to be produced for the animal and 
DFM industries. 

The practice of supplementing direct fed microbial and prebiotic additives to domestic animals during growth is becoming more widespread in food animal production. Benefi cial effects particularly in cattle, pigs and poultry including improved general health, foodborne pathogen reduction, more effi cient food utilization, faster growth rate and increased milk and egg production continue to be reported. The success associated with direct fed microbial and prebiotic applications in multiple species ensures their continued commercialization and widespread use of such 
additives. However, several fundamental questions remain. It appears that early establishment and retention of an ecological balance in the gastrointestinal tract is an important fi rst step for an external biological additive to be effective in young animals. Therefore, it is possible that the effectiveness of direct fed microbials and prebiotics in some animal species may only be an indirect consequence of speeding up the establishment of the dominant microfl ora characteristic of the adult viii Preface gastrointestinal tract. Consequently an understanding of the key processes during establishment of microfl ora in the gastrointestinal system that lead to the subsequent fermentation characteristics and ecological balance exhibited by the highly protective microfl ora is needed. Identifying these processes should lead to continued improvement in the effectiveness of available commercial products. Several 
additional areas of future research directions are also likely needed for further development and implementation of these biologicals. 

 A critical area that is now becoming possible is the rapid identifi cation in vivo of characteristic microbial profi les to confi rm successful establishment. Such techniques involve incorporation of molecular fi ngerprinting of both externally introduced cultures as well as the indigenous gastrointestinal microfl ora. This may also potentially help to achieve a better understanding of the mechanism(s) required for successful selection and optimization of direct fed microbials and prebiotics. In addition, this will provide insight into environmental factors that may play a role in the ability of direct fed microbials to limit pathogen transmission. Other arenas in which direct fed microbials and prebiotics may be important are in limiting establishment of pathogens in older animals which possess a more mature and developed gut microfl ora and need removal of pathogens already colonized in animal gastrointestinal tracts. Here success will be dependent on a much more complete picture of gastrointestinal microbial ecology and may include organisms which have been overlooked when typical direct fed microfl ora have been identifi ed and characterized. In addition, modeling of microbial interactions in the gastrointestinal tract may be important to identify common factors within the complex matrix of the microbial consortium which help to serve as a barrier to prevent pathogens from coexisting with these microorganisms. Continued research on direct fed microbials and prebiotics in general should markedly expand their commercial applications.