The chemical composition of a star is not constant throughout its life. It changes due to different evolutionary processes such as atomic diffusion between the main sequence and the sub-giant brach, the first dredge-up along the sub-giant branch, and extra-mixing taking place after the luminosity bump on the red giant branch. All these effects are hard to measure observationally, since a fairly large sample of stars belonging to the same stellar population and in different evolutionary stages is required. Being built up of a single stellar population, open clusters are therefore perfect laboratories for the study of stellar evolution. We present the well known old open cluster M67 as a test-bench for different evolutionary effects using high-resolution spectroscopic data from APOGEE and Gaia-ESO. We compare the observational results to predictions from theoretical models and discuss them with respect to broader implications such as the [C/N]-age relation, recently proposed for the age-dating of field stars.