  Searching for the physical mechanism that can excite the coherent radio
emission in pulsars is still an enigmatic problem. A wealth of high quality
observations exist, which over the years have been instrumental in putting
stringent constraints to pulsar emission models. In this article we will
discuss the observational results that strongly suggests that pulsar radio
emission is excited by coherent curvature radiation. We will also mention
issues that remain to be resolved.
