The application of FRET-based molecular biosensors provided confirmation of the central model of Ran GTPase function and led to important new insights into its physiological role. In many fields of cell biology, methods employing FRET are a standard approach that is becoming increasingly accessible due to advances in instrumentation and available fluorophores. However, the optimal design of a FRET sensor remains to be the cornerstone of any successful FRET application. Utilizing the recent literature on FRET applications and our studies on Ran, we outline the basic considerations involved in designing molecular FRET sensors. We point to several broadly applicable principles that were used in many different FRET sensors that can detect a wide range of molecular events. Using the FRET sensors for Ran that we created as examples, we then focus on the practical aspects of FRET assays. We describe the preparation of a bipartite FRET sensor consisting of ECFP-Ran and EYFP-importin beta and its validation as a reporter for FRET-based high throughput screening in small molecule libraries. Finally, we review the design and optimization of monomolecular FRET sensors that monitor the RanGTP-RanBP1 interaction, and of sensors detecting the RanGTP-regulated importin beta cargo release.