Many important issues in community ecology revolve around the interpla
y of competition and predation. Species that compete may also be locke
d in predator-prey interactions, a mixture of competition and predatio
n known as ''intraguild predation'' (IGP). There is growing evidence f
or the importance of IGP in many natural communities, yet little forma
l ecological theory addresses this particular blend of interactions. I
n this article, we explore the consequences of incorporating IGP into
standard models of exploitative competition and food chains (a general
resource-consumer model, a Lotka-Volterra food chain model, and Schoe
ner's exploitative competition model). Our theoretical analyses sugges
t a general criterion for coexistence in IGP systems: the intermediate
species (the prey in intraguild predation) should be superior at expl
oitative competition for the shared resource, whereas the top species
(the predator) should gain significantly from its consumption of the i
ntermediate species. Along gradients in environmental productivity, co
existence is most likely at intermediate levels of productivity. Analy
ses of the models reveal the potential for alternative stable states i
n systems with IGP; these are particularly likely if the top predator
gains little benefit from consuming the intermediate predator. We furt
her show that IGP can lead to unstable population dynamics, even when
all pairwise interactions are inherently stable and each species can i
ncrease when rare. Persistent, strong IGP raises a puzzle of species c
oexistence, particularly in productive environments. We conclude by co
mparing IGP with related community modules (i.e., food chains, exploit
ative competition, apparent competition) and discussing mechanisms tha
t should foster coexistence in systems with strong IGP.