Macrophages reside in all tissues as resident populations and as immigrants recruited in response to tissue injury, inflammation or pathogen invasion. Under normal conditions, macrophages contribute to tissue homeostasis and provide innate immune surveillance. Both macrophages and their progenitors, bone marrow-derived monocytes, constitutively express the tumor necrosis factor receptor superfamily member, CD40, and are capable of a robust response to CD40 ligation resulting in the induction or enhancement of expression of genes with a predominantly pro-inflammatory function. CD40 signaling in macrophages in the context of host responses to pathogens plays a crucial role in host defense. However, macrophage responses to CD40 ligation in the context of autoimmune and cardiovascular disease contribute to disease pathogenesis. In this review, we discuss the role of CD40 in both protective and destructive processes, including the signaling pathways engaged and the factors capable of modulating CD40 signal transduction.