This White Paper examines four key challenges that must be addressed to make progress towards developing ethical artificial intelligence (AI) systems. These chal- lenges arise from the way existing AI systems reason and make decisions. Unlike hu- mans, AI systems only consider the objectives, data and constraints explicitly pro- vided by their designers and operators. They possess no intrinsic moral awareness or social context with which to understand the consequences of their actions. To build ethical AI systems, any moral considerations must be explicitly represented in the objectives, data and constraints that govern how AI systems make decisions.