Prepare Marinade: In a bowl or zip-top bag, mix olive oil, lemon juice, zest, garlic, herbs, salt, pepper (and yogurt if using).
Marinate Chicken: Add chicken fillets, coat well, seal or cover, and refrigerate for 1–2 hours.
Preheat Grill: Heat grill to medium-high (~400°F).
Grill Chicken: Remove chicken from marinade and let excess drip off. Grill 6–7 minutes per side, or until internal temperature reaches 165°F (74°C).
Rest & Serve: Let rest for 5 minutes before slicing. Serve with grilled vegetables, couscous, or a fresh salad.