This paper describes the current best practices and critical steps of developing metallurgical test plans and the final important step to process the design of a circuit. No two deposits are the same, and developing test plans that address the geology and ore domains is a complex task. Sample selection and ore characterisation to understand the metallurgical response of an ore are only the start. Invariably, test plans need to be updated based on the results received. The testwork needs to be considered as a project with the scope of work, schedule, cost, quality, safety and environment the new norms required. It needs to be managed using the best project management knowledge available. The data interpretation of the results is the most difficult part and requires personnel skilled in the art as well as benchmarking and discussing with vendors and reference to existing projects. Project failure analysis often highlights where gaps in knowledge existed and the interpretation of the metallurgical results was deficient. In some cases, this may require piloting of the process to improve process confidence, reduce risks and provide engineering data. Often this is all summarised in a metallurgical summary report. Process design is the final step and has its own challenges with regard to not being too conservative but ensuring the design is functional and operable. A number of examples are provided highlighting the lessons learned.