Several catalyst and reactor variables have been observed to exert a strong influence on long-chain branch (LCB) formation during ethylene polymerization while using Phillips Cr/silica or metallocene type catalysts. These include ethylene concentration, solution versus slurry process, catalyst calcination temperature, Cr and Ti loading, the presence or absence of cocatalysts or poisons, catalyst porosity and nano-structure, and active site Lewis acidity and/or sterics. These LCB responses are reviewed and various possible explanations for each are suggested. One conclusion from this study is that LCB formation is much more selective than is usually envisioned from the simple inter-molecular mechanism of random macromer insertion that is widely accepted, and analogous to comonomer insertion. In fact, some of these variables, such as the catalyst porosity, can swing LCB content over an extremely broad range under otherwise identical catalyst and reaction conditions. To account for such unexpected behavior one or more modifications of the mechanism of LCB formation is needed.