Preheat your grill to medium-high heat.
In a medium bowl, combine the parsley, cilantro, red wine vinegar, olive oil, garlic, red pepper flakes, salt, and black pepper. Mix well to create the chimichurri sauce.
Reserve about 1/4 cup of the chimichurri sauce for serving.
Place the chicken breasts in a large resealable plastic bag or a shallow dish. Pour the remaining chimichurri sauce over the chicken, ensuring it is well coated. Marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more flavor.
Remove the chicken from the marinade and let any excess drip off. Discard the used marinade.
Grill the chicken for 6-7 minutes per side, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for 5 minutes.
Serve the grilled chicken with the reserved chimichurri sauce drizzled on top.