The book presents recent results and new trends in the theory of fluid mechanics. Each of the four chapters focuses on a different problem in fluid flow accompanied by an overview of available older results.The chapters are extended lecture notes from the ESSAM school "e;Mathematical Aspects of Fluid Flows"e; held in Kacov (Czech Republic) in May/June 2017.The lectures were presented by Dominic Breit (Heriot-Watt University Edinburgh), Yann Brenier (Ecole Polytechnique, Palaiseau), Pierre-Emmanuel Jabin (University of Maryland) and Christian Rohde (Universitat Stuttgart), and cover various aspects of mathematical fluid mechanics - from Euler equations, compressible Navier-Stokes equations and stochastic equations in fluid mechanics to equations describing two-phase flow; from the modeling and mathematical analysis of equations to numerical methods. Although the chapters feature relatively recent results, they are presented in a form accessible to PhD students in the field ofmathematical fluid mechanics.