Design and sustainability are enmeshed. Many visions of a sustainable future assume large-scale changes in human behaviour, in tandem with scientific advances. A major component of this is design which relates to people’s actions: the design of products, services, environments and systems plays an important role in affecting what people do, now and in the future. This has become known, in recent years, as design for behaviour change, behavioural design, or in the case of specific focus on sustainability, design for sustainable behaviour. However, planning anything around human action is bound up with assumptions and – in the case of much work around design for behaviour change – determinism. Design which adopts a singular, linear vision of the future, and future human behaviour, does not deal well with the complexities of humanity, culture and society. How can we ‘plan’ for sustainability while embracing this complexity? Is it possible to use speculation and reflection to think through some of the potential consequences and side effects? In this chapter, we introduce questions that designers interested in futures, sustainability and people’s actions can use to explore speculative approaches to future human behaviour.