The Neutron Star Interior Composition Explorer (NICER) has been in operation from the International Space Station since June 2017. Its primary science goals is to measure the radius and mass of millisecond pulsars, fast-spinning X-ray emitting neutron stars. To achieve this, a handful of these pulsars have been observed by NICER, totalling more than 1 Megaseconds each. By accurately modelling their phase-energy resolved light curves, originating from their hot polar caps, we can obtain measurements of the neutron stars’ masses and radii. In this presentation, I will summarise the observational data and the modelling techniques applied to our first target, the millisecond pulsar PSR J0030+0451. In addition to the measured radius and mass (a first for an isolated neutron star), I will also show some of the other surprises resulting from our analyses. These results place interesting constraints on the equation of state of dense matter, a missing piece of nuclear physics, but also on our understanding of pulsars magnetospheres. In this talk, I will also present other interesting results on pulsars from the NICER mission.