A Thesis submitted to the Faculty of Graduate Studies and Research in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Mathematics, University of Regina. x, 132 p. For any simple graph G on n vertices, the (positive semi-de nite) minimum rank of G is de ned to be the smallest possible rank among all (positive semi-de nite) real symmetric n × n matrices whose entry in position (i; j), for i x j, is non-zero if ij is an edge in G and zero otherwise. Also, the (positive semi-de nite) maximum nullity of G is de ned to be the largest possible nullity of a (positive semi-de nite) matrix in the above set of matrices. In this thesis we study two graph parameters, namely the zero forcing number of G, Z(G), and the positive zero forcing number of G, Z+(G), which bound the maximum nullity and the positive semi-de nite maximum nullity from above, respectively. With regard to the zero forcing number, we introduce some new families of graphs for which the zero forcing number and the maximum nullity are the same. Also we establish an equality between the zero forcing number and the path cover number for a new family of graphs. In addition, we establish a connection between the zero forcing number and the chromatic number of graphs. With regard to the positive zero forcing number, we introduce the concept of forcing trees in a graph and we establish a connection between the positive zero forcing number and the tree cover number. Also we study families of graphs for which these parameters coincide. In addition, we provide some new results on the connections of this parameter with other graph parameters, including the independence number and the chromatic number of G. A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy *, University of Regina. *, * p. Student yes