Crystallisation, the spontaneous arrangement of molecular building blocks into ordered solid particles, is a fascinating phenomenon. Understanding the dynamic, molecular-scale processes that underlie crystal nucleation and growth holds the key to designing the production of specific crystalline materials The ability to induce crystallisation how, when and where we want it is key to material synthesis. Such capabilities will transform industrial and environmental sectors, including healthcare, formulated products, oil and gas, water, mining and advanced materials. This Discussion focuses on the following four themes: Understanding crystal nucleation mechanisms: where do we stand? Growing crystals by design Controlling polymorphism Learning Lessons from Nature – the future of biomimetics