This report provides a discussion on the relative importance of electronic and steric effects in catalysis. More specificially, electronic effects that arise from phosphorus based supporting ligands in palladium catalysed cross-coupling and rhodium catalysed hydroformylation are reviewed and analysed. The report identifies certain trends that emerge from the literature data, and also points out that these effects can sometimes be subtle, or an indirect effect on the catalytic reaction rather than changing the rate of any of the key steps in the catalytic cycle directly.