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The study of light fields near nanophotonic structures continually reveals new fundamental features of light–matter interactions on the nanoscale, driving advances in fields ranging from nonlinear and quantum optics to biosensing. Here, we have succeeded in separately mapping the electric and magnetic fields. This allows us to present the first fully three-dimensional maps of the in-plane electromagnetic near fields of a photonic crystal waveguide, in experiment and theory. In these fields, we identify and study the spatial evolution of infinitesimally small optical entities: optical singularities. We discuss the topological properties of the local light fields in the vicinity of the singularities and show that the trajectory traced by each singularity through three-dimensional space is distinct. These results are an important step toward understanding the behavior of light at the nanoscale, opening up new avenues for on-chip control and detection of nanoscopic or quantum objects with structured light fields.