The symbioses between many Pooideae grasses and Neotyphodium fungal endophytes are mutualistic relationships with both partners gaining benefits. The more we learn from the ongoing research conducted throughout the world, the more we realize just how wonderfully complex is the association between these organisms from two kingdoms. Although there are many direct implications of the symbiosis-effects on insects, grazing animals, and plant survival-the understanding of how the two mutualists interact provides opportunities to use these fungal endophytes better to enhance forage production as well as to stimulate further basic research. The purpose of this review is to provide a pictorial representation of the nature of the symbioses between tall fescue and Neotyphodium coenophialum (Morgan-Jones and Gams) Glenn, Bacon, and Hanlin, and also that of other asexual Neotyphodium species and their close sexual relatives, the Epichloë species, with their respective host grasses. For detailed information on tall fescue and Neotyphodium endophytes (see Chapter 2). For an alternate view on the nature of the endophyte grass symbiosis, see Faeth and Sullivan (2003), who described some native North American grasses in which the asexual endophytes are parasitic.