In this paper, we propose optimal (w.r.t, the number of robots) solutions for the deterministic exploration of a grid shaped network by a team of k asynchronous oblivious robots. In more details, we first show that no exploration protocol exists with less than three robots for any grid with more than three nodes, less than four robots for the (2, 2)-Grid, and less than five robots for the (3, 3)-Grid. Next, we show that the problem is solvable using only 3 robots for any (i, j)- Grid, provided that j > 3. Our result is constructive as we present a deterministic algorithm that performs in the non-atomic CORDA model. We also present specific deterministic protocols for the (3, 3)-Grid using five robots.