There are various ways to solve for the external mode (depth integrated) velocities in MOM. The purpose of this chapter is to describe the numerical issues involved. Many of the theoretical issues were discussed in Chapter 4. Starting this chapter are some comments concerning the main points to be considered when choosing a particular method. Additionally, within the discussion of each method, certain caveats, which are mostly suggestive and not rigorous, are mentioned. In general, stability of the external mode in ocean modeling tends to be the very sensitive, especially in global models with realistic geography and topography. A great deal of energy has gone into deriving various methods for solving the external mode. The present methods are not perfect, and development continues.