The main aim of the experimental approach in research is to discover causal relationships between variables. This article describes the concept of causality and discusses common and important aspects and factors related to causality, manipulation and control. The four principles which underpin the inference of causality in experiments are highlighted and the role of 'control' in achieving the aim of experimental studies is discussed. Some methods of control such as manipulation, randomization, matching subjects, and holding extraneous variables constant are considered. Experimental designs are underpinned by these concepts. Indication is given of the threats to internal validity, which will be discussed in detail in the next article in this series.