Mix together the olive oil, lime juice and zest, garlic, cilantro, salt and pepper in a large bowl. Add chicken and cover tightly with plastic wrap. Put in the fridge and allow to marinate at least one hour, or overnight. (Or, for a great freezer meal put the marinade and chicken in a freezer bag and put in the freezer until ready to use. To use thaw and cook as normal.)
Heat grill or grill pan (or, sadly like me, just a regular pan) over medium-high heat. Add the chicken, working in batches if necessary (don't crowd the pan). Cook for about 6 minutes on each side depending on thickness, or until done.
Recipe by Katherine Martinelli at http://www.katherinemartinelli.com/blog/2011/secret-recipe-club-cilantro-lime-chicken/