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 subgiant branch, the first dredge-up along the subgiant and lower red 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 a study of the effects of atomic diffusion on the stars of the well-known old open cluster M67 using high-resolution spectroscopic data from the Gaia-ESO Survey. We compare the observational results to predictions from theoretical models and discuss them with respect to broader implications. In fact, atomic diffusion sets the resolution achievable by chemical tagging methods, since stars cannot be traced back to their cluster of origin with a precision higher than the variation in surface chemical composition caused by evolutionary effects. Furthermore, we will present the results of a study on the chemical composition of blue straggler stars and evolved blue straggler stars in M67 using data from APOGEE DR14. We will discuss the implications of these results in the investigation of the formation scenario and the evolution of the objects under study.