Analysing interactions between niches and regimes is critically important for understanding sustainability transitions. What complicates interaction is the fact that sustainability is often understood and pursued differently in regimes compared to niches. That means the aims and criteria for innovation can be different on either side of the interaction. A paradox arises in which interaction will be easiest when there is already good alignment between niche and regime sustainability criteria, but such alignment will by definition not demand very great changes in the regime nor empower more radical niche experiments. In practice, four different interactions coexist: differentiation; co-option; hybridisation; and criticism. These interactions work in both directions, can be interdependent upon one another, and influence wider change processes over time. It becomes problematic to conceive reconfiguration as a single transition process originating in niches and linking to regimes. This is illustrated with an example that contrasts sustainability in the automation regime of the Fourth Industrial Revolution (4IR) with a niche space that we call post-automation. In becoming attentive to niche-regime interactions, so the politics of sustainable transitions becomes clearer.