To start, remove rosemary from the stems and chop the herb.
In a large bowl add oil, chopped rosemary, balsamic vinegar, salt, black pepper and garlic.
Now, you can finely mince the garlic but I recommend grating the garlic in the marinade. Give the marinade a good mix.
Next, toss in the chicken and mix well. Cover bowl and place in the refrigerator if you are not cooking the chicken within the next hour. If you plan to cook much later be sure to remove the chicken before cooking an hour before grilling so the chicken isn't ice cold.
When chicken has marinated, fire up the grill. You can use a charcoal or gas grill. Either is fine but you want to turn the heat up to at least 400ºF. A great tool to use is a thermal thermometer. I use a charcoal grill without a thermometer so I always have one handy with a probe as well. Once the grill is lit and the grates are hot add the chicken thighs from the marinade onto the grill. You will get flare ups very quickly but move the chicken away from the flare ups until they stop. This means do not crowd the grill.
Allow the chicken to cook on both sides flip every 3 minutes or so. Do not close the lid on the grill when making this recipe.
Check the chicken for temperature around 165ºF* and remove the chicken from the grill. Allow the grilled boneless chicken thighs to rest a few minutes before serving.