  Radio observations from normal pulsars indicate that the coherent radio
emission is excited by curvature radiation from charge bunches. In this review
we provide a systematic description of the various observational constraints on
the radio emission mechanism. We have discussed the presence of highly
polarized time samples where the polarization position angle follow two
orthogonal well defined tracks across the profile, that closely match the
rotating vector model in an identical manner. The observations also show the
presence of circular polarization, with both the right and left handed circular
polarization seen across the profile. Other constraints on the emission
mechanism is provided by the detailed measurements of the spectral index
variation across the profile window, where the central part of the profile,
corresponding to the core component, has a steeper spectrum than the
surrounding cones. Finally, the detailed measurements of the subpulse drifting
behaviour can be explained by considering the presence of non-dipolar field on
the stellar surface and the formation of the Partially screened Gap (PSG) above
the polar cap region. The PSG gives rise to a non-stationary plasma flow, that
has a multi-component nature, consisting of highly energetic primary particles,
secondary pair plasma and iron ions discharged from the surface, with large
fragmentation resulting is dense plasma clouds and lower density inter-cloud
regions. The physical properties of the outflowing plasma and the observational
constraints lead us to consider coherent curvature radiation as the most viable
explanation for the emission mechanism in normal pulsars, where propagation
effects due to adiabatic walking and refraction are largely inconsequential.
