Polymer particle growth in catalytic ethylene slurry polymerizations with SiO2-supported metallocene and post-metallocene catalysts was monitored online using a Lasentec FBRM probe inserted into the stirred reactor. FBRM enabled the online monitoring of particle numbers and size distributions. Trend analyses provided time-resolved information on selected variables of the particle growth processes. FBRM was combined with simultaneous ethylene mass flow measurements in order to distinguish between ideal particle growth and more complex growth processes involving particle fragmentation and aggregation. Additional use of a Lasentec PVM video microscopy probe during ethylene polymerization on a MgCl2-supported Ziegler catalyst enabled the online visualization of PE particles, complementing the data generated by FBRM online monitoring.