In many settings, key populations (in particular men having sex with other men, sex workers and drug users) are overwhelmingly affected by HIV and many programs are focusing on them. Due to epidemiological modelling and policy planning, these groups are too often seen as socially homogeneous groups rather than heterogeneous categories with fuzzy outlines. Identities, social life, sexual practices, exposure to HIV varies within these groups, in particular between the core and the edges. Their sexual networks are also connected to individuals not perceived as “key populations”: female partners of bisexual men, regular partners and clients of sex workers, sexual partners of drug users… Could we better conceptualize the edges of key populations? What do we know about them? What is their role in the dynamic of HIV epidemics? How can we include them in the HIV response? Are some interventions more relevant to reach them?