What children do and say provides us with important information about their development and wellbeing. However, there is only an indirect relationship between the former and the latter so that observing children alone cannot help us understand the underlying mechanisms. Experimental designs are there to explore specific hypotheses and examine the cause and effect relationship of different phenomena. This chapter will focus on how behavioural, experimental and standardised psychological tasks are used within experimental designs. In the first section, we will discuss the use of standardised tasks, focusing on selecting the appropriate task, issues that relate to the administration of the task, and how to correctly interpret the scores obtained from standardised tests. In the second part of the chapter, we will focus on experimental tasks. More specifically, we will discuss the advantages and disadvantages of designing your own experimental tasks and what confounding factors should be taken into account during the design stage. Next, the issue of validity as well as general issues when using tasks with children will be explored. Throughout the chapter, we will use examples from research with children who have neurodevelopmental disorders, not only because it includes our own research, which we know best, but more importantly, because working with children with additional needs reinforces certain issues when using experi - mental and standardised tasks.