Table of Contents -- 1. Introduction -- 2. Finite dimensional normed spaces -- 3. The best nonoscillation constant for real systems -- 4. Nonoscillation criteria for complex systems -- 5. The Euler-Lagrange equations -- 6. Two dimensional cases -- 7. The constant c[sub(p1,p2)](n) and C[sub(p1,p2)](n) -- 8. Disconjugacy of real differential equations -- 9. Multipoint boundary value problems for nonlinear systems -- References