As mammals, primates possess the mammalian characteristics of endothermy (internal regulation of body temperature, often known as warm-bloodedness), bearing live young (placental), and feeding their young with milk produced by mammary glands. 
Not all primates possess the same characteristics—there is no unique characteristic that defines a primate. Most shared characteristics and trends are not derived but instead are a retention of ancestral features, which also adds difficulty to classifying primates. Many of these characteristics are behavioral, or depend on soft tissue anatomy.