-- During the last 20 years, complexity has been an interesting topic that has been investigated in many fields of science, such as biology, neurology, software engineering, chemistry, psychology, and economy. A survey of the various approaches to understand complexity has lead sometimes to a measurable quantity with a rigorous but narrow definition and other times as merely an ad hoc label. In this paper we investigate the complexity concept to avoid a vague use of the term `complexity' in workflow designs. We present several complexity metrics that have been used for a number of years in adjacent fields of science and explain how they can be adapted and use to evaluate the complexity of workflows.