Markovnikov is best known for the Markovnikov Rule which he developed in 1869. The rule is useful for predicting the molecular structures of products of addition reactions. Since he never published in any language besides Russian his work remained largely unknown until 1889.
Markovnikov also contributed to organic chemistry by finding carbon rings with more than six carbon atoms, a ring with four carbon atoms in 1879, and a ring with seven in 1889.