Queuing models with customers' impatience have become the hot topic of research in the current era. We normally encounter such queuing systems in grocery stores, banks, hospitals, computer-communication networks etc. Customers' impatience in case of revenue generating businesses leads to loss of revenue while customers' impatience (packet loss, frame loss etc.) in case of computer-communication networks leads to the loss of valuable information. Hence, the study of queuing systems with impatient customers assumes a lot of significance. In this book we have developed and analysed a number of Markovian queuing systems with customers' impatience and retention, and have derived their transient solutions. The probability generating function technique and Runge-Kutta method of fourth order are employed for the transient analysis. Some important measures of performance like mean, variance, average reneging rate, and average retention rate are derived and discussed. A number of numerical examples are provided in different chapters of the book which deal with the sensitivity analyses and some comparisons.