A Lagrangian-based method of analysis is introduced to examine the space-time structure and dynamics of extratropical cyclogenesis. the three-step method involves the calculation of extensive ensembles of trajectories, the evaluation of the Lagrangian time-trace of physical variables along these paths, and the application of objective selection criteria to identify significant Lagrangian structures. the approach can provide a qualitative depiction of, quantitative information about, and dynamical insight into, key features of mid-latitude cyclones. In this, the first of a two-part study, examples are given of the application of the method. A case-study approach has been adopted and used to identify coherent ensembles of trajectories (CETs) whose dynamics are seminal to an event of Atlantic cyclogenesis, to trace the origin of subsynoptic potential vorticity anomalies, and to analyse stratosphere-troposphere exchange accompanying extratropical cyclogenesis. In addition, a statistical investigation has been made of the climatology of CETs in the northern hemisphere for one particular winter month. Some consideration has been given to the shortcomings and potential of the method, and its complementarity with other approaches.