Overseeding is when grass seed is applied to an existing turf. It is usually performed in the fall, during core aeration and organic fertilizing. By putting down seed at this time, you have an opportunity to get better ground/seed contact. Because core aeration punches holes in the thatch and soil, this will allow for more contact, which is important for seed germination and survival. Also the organic fertilizer does not harm the seed, chemical fertilizer will burn the seed and young grass (unless a starter fertilizer is used). This can be done in the spring if needed. We like to use a high quality seed blend which usually consists of three or more varieties of grass. One seed might do better in shade, while another variety might do better in the sun. This is one reason to use a variety of seed. Another is that they are cultivating new varieties all the time. While some of the old varieties are still good, some of the new might have some better characteristics, like stronger root system, more drought tolerant, disease resistant, or better tolerance to traffic.