Agent-based simulation (ABS) has been used widely in various application areas. This paper provides a tutorial on ABS. In addition to the theoretical foundations, such as the definitions of key ABS concepts and the structure of an ABS model, this tutorial also explains the technical aspect, such as how an ABS model is implemented using a software tool and how ABS is used to solve a problem. The software tool used in this tutorial is Repast. The last part of the tutorial discusses the challenges that need to be addressed in order to increase the confidence of decision-makers, who may not be familiar with computer programming, when using ABS to help them make decisions. The challenges include conceptual model representation, validation and participatory modelling.