Many pathways involved in innate immunity in plants, insects, and mammals also play roles in host development. This genetic pleiotropy could constrain the ability of immune systems to rapidly evolve in response to selective pressure from microbes because adaptive evolution could negatively affect other developmental processes and thus host fitness. Does pleiotropy constrain adaptive evolution, and if so why is it maintained over evolutionary time? We analyzed statistical signatures of selection on non-pleiotropic and pleiotropic immune genes in insects, discovering that pleiotropic genes do exhibit stronger signatures of evolutionary constraint. To understand why pleiotropy might be maintained despite these apparent costs, we used an agent-based model of immune signaling network coevolution to discover that pleiotropy can facilitate the evolution of robust inducible immune responses, providing an explanation for the maintenance of pleiotropy in immune systems (and traits with pleiotropic genetic architecture) over evolutionary time. I will also discuss some recent ideas on bet-hedging in immune systems, including trade-offs associated with relying on plasticity in infection-induced immune responses.