Polymeric micelles and polymersomes self-assembled from amphiphilic and double hydrophilic block copolymers (DHBCs) offer great promise as smart nanocarriers of chemotherapeutic drugs, genes, and imaging or contrast agents. Specifically, fluorescent polymeric assemblies and nanoparticles render possible the in situ tracking of intracellular transport, in vivo circulation characteristics, and biodistribution of drug-loaded or conjugated nanocarriers. The introduction of fluorescence resonance energy transfer (FRET) processes into polymeric assemblies and nanoparticles can further enhance relevant functions due to the fact that the tracking can be conducted in a ratiometric manner to effectively exclude background interference and enhance spatiotemporal detection resolution. Moreover, if the FRET efficiency is responsive to the variation of solution conditions such as pH value, temperature, metal ions, glucose, and tissue-specific enzymes by taking advantage of the responsiveness of polymeric matrix, sensing of the external microenvironment will also be achieved. This article highlights recent developments of fluorescent polymers, polymeric assemblies and nanoparticles involving FRET events, in which the modulation of FRET efficiency can be achieved by tuning the spatial distribution of fluorescent donors and acceptors.