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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�� 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.