Most scholars now agree that process tracing, as a distinct social science method, involves tracing causal mechanisms using in-depth case studies. Studying causal mechanisms shifts the analytical attention from causes and outcomes to the causal process that links causes and outcomes together. However, while they are widely used, statements about causal mechanisms are also the least understood type of causal claim in the social sciences. This short essay discusses two competing approaches to studying causal mechanisms (a counterfactual-based and systems understanding of mechanisms), arguing that adopting a systems understanding of mechanisms results in research designs that produce actual evidence of the process we are attempting to trace instead of hypothetical ‘what ifs’ or weak comparisons.