The book is an introduction to the experiments and theoretical study of the horizontal gas jets and vertical plunging liquid jets that is great interest in environment and many industrial applications, especially in the safety system of nuclear power plants. The material of this book is appropriate for undergraduate students in nuclear, mechanical, environmental and hydraulic engineering. It will be assumed that the students have an introductory course in fluid mechanics and that they are familiar with the basic principles of fluid mechanics: continuity, momentum, and buoyancy principles. The book will first develop an introduction on the basic principles of jets and its different flow configurations. The methodology entails setting up and executing two different experiments on the horizontal gas jets and vertical plunging liquid jets and the structure of the book is divided into the following main parts; Part 1. Experimental study of the buoyant gas jets injected horizontally in water ambient. Part 2. Developing an integral model to predict the gas jet trajectories and the model validation. Part 3. Experimental study of the vertical plunging liquid jets on a free water surface.