The year 2013 marked the centenary of the paper of Leonor Michaelis and Maud Menten (Michaelis and Menten, 1913), and the 110th anniversary of the doctoral thesis of Victor Henri (Henri, 1903). These publications have had an enormous influence on the progress of biochemistry, and are more often cited in the 21st century than they were in the 20th. Henri laid the groundwork for the understanding of enzyme mechanisms, but his experimental design was open to criticism. He reached essentially correct conclusions about the action of invertase, but he took no steps to control the hydrogen-ion concentration, and he took no account of the spontaneous mutarotation of the glucose produced in the reaction. Michaelis and Menten corrected these shortcomings, and in addition they introduced the initial-rate method of analysis, which has proved much simpler to apply than the methods based on time courses that it replaced. In this way they defined the methodology for steady-state experiments that has remained standard for 100 years.