We present a new approach for analysing structural properties of time series from complex systems.Starting from the concept of recurrences in phase space, the recurrence matrix of a time series is interpreted as the adjacency matrix of an associated complex network which links different points in time if the considered states are closely neighboured in phase space.In comparison with similar network-based techniques, the new approach has important conceptual advantages and can be considered as a unifying framework for transforming time series into complex networks that also includes other existing methods as special cases.