Regarding an oil-&-gas reservoir, there are only two fundamental questions that constitute the basis of exploration and production of hydrocarbons:

• How much can we expect to earn?
• How much could we lose?

Answering these two questions correctly is crucial to making the right decision, which arises immediately after having spent tens of millions of dollars to acquire data on a lease and rights before again spending hundreds of millions of dollars to develop the oil field. Such a decision has to be taken on the basis of a numerical model of the subsurface, integrating all the data and knowledge that have been collected and interpreted by a series of geoscientists, from the geophysical interpretation to the flow simulator. This course presents several types of approaches that can be used to build such models, while pointing out their pros and cons.