The rational understanding and design of catalysts pose major challenges to chemists. While catalysts are involved in around 90% of industrial chemical processes, their discovery and development are usually based on screening and serendipity. Here, we show through a detailed analysis of the NMR chemical shift that the activity of olefin polymerization and oligomerization catalysts is directly related to the chemical shift of the carbon atom bound to the metal center. This relation is traced to specific frontier molecular orbitals, which induce π-character in the metal-alkyl bond, thereby favoring insertion. This result not only reveals a surprising analogy between olefin polymerization and metathesis, but also establishes chemical shift as a predictive descriptor for catalytic activity in these industrially relevant processes.