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¡¡¡¡Entry-exit decisions apply to numerous practical problems. For example£¬ when to extract oil and when to stop the extraction£¬ when to issue a new policy and when to end it£¬ and when to let a kind of product enter a market and when to let the product exit the market£¬ etc. Therefore£¬ entry-exit decision problems attract a lot of researchers.¡¡¡¡There are three approaches to study entry-exit decisions£¬ namely£¬ real option£¬ pure probability and optimal stopping. In this monograph£¬ we appeal to optimal stopping to deal with entry-exit decisions. The main reasons are as follows. On the one hand£¬ in the real option framework£¬ the regularity of payoff functions is a priori assumed£¬ while the optimal stopping approach intends to prove it. On the other hand£¬ although the pure probability and optimal stopping approaches are both to solve a optimal stopping problem£¬ we have to calculate density functions of some stopping times if applying the pure probability approach£¬ which is not easy£¬ whereas the optimal stopping one avoids such calculations.¡¡¡¡We aim to obtain closed-form solutions of optimal entry-exit decisions for the cases£º costs depending on underlying processes£¬ implementation with delay£¬ and underlying processes following geometric Levy processes. In addition£¬ we provide a complete theory for optimal stopping problems with regime switching£¬ and use it to solve an exit problem.