Techno-economic optimisation plays a pivotal role in the design and operation of microgrids by balancing technical performance with financial viability. This chapter presents a structured optimisation framework that supports the planning, temporary islanded operation, and steady-state functioning of microgrids. It outlines how distributed energy resources (DERs), energy storage systems (ESSs), and electric vehicle supply equipment (EVSE) can be optimally sized and sited to meet a range of objectives—such as minimising lifecycle costs, enhancing reliability, and reducing emissions—subject to technical, economic, environmental, and social constraints. The framework introduces multiple objective functions (e.g., net present value, total lifecycle cost, reliability indices) and constraints (e.g., voltage limits, power quality, thermal capacity, social impacts), and incorporates advanced mathematical solvers, such as the interior-point optimiser (Ipopt), to solve unbalanced optimal power flow problems. By modelling generation profiles, system degradation, and energy demand growth, the framework enables accurate, long-term planning under uncertainty. Additionally, it incorporates stakeholder-specific metrics, including job creation, community engagement, and energy equity, to guide socially sustainable outcomes. The chapter concludes with a validation methodology using worst- and best-case scenarios and Monte Carlo simulations, ensuring robust, context-specific microgrid designs tailored for both utility and customer benefit.