In this paper we consider the implications of habits for optimal monetary policy, when those habits either exist at the level of the aggregate basket of consumption goods (‘superficial’ habits) or at the level of individual goods (‘deep’ habits: see Ravn et al., 2006). External habits generate an additional distortion in the economy and create new trade-offs for optimal policy, as the policy maker does not respond as aggressively to technology shocks in order to avoid exacerbating the habits externality. This can dramatically affect both the parameterization of optimal simple rules, as well as their determinacy properties. These effects are particularly strong when habits are of the deep kind.