1. Introduction and Preliminaries.- 2. Some Arithmetic and Analysis in Qp; Derivatives in Ultrametric Analysis.- 3. Ultrametric Functional Analysis.- 4. Ultrametric Summability Theory.- 5. The N?rlund and the Weighted Mean Methods.- 6. The Euler and the Taylor Methods.- 7. Tauberian Theorems.- 8. Silverman-Toeplitz Theorem for Double Sequences and Double Series.- 9. The N?rlund Method and the Weighted Mean Method for Double Sequences.