This work presents lecture notes based on the Conference Board of Mathematical Sciences-National Science Foundations Regional Research Conference lectures, delivered in 2014 at Oklahoma State University. Discussion centers on classical solutions of certain partial differential equations related to hydrodynamics. The material provides a unified mathematic approach for local existence and uniqueness based on solution paths, along with global regularity results for nonlocal dissipative active scalar equations. Coverage encompasses Lagrangian and Eulerian descriptions of hydrodynamic systems, spaces and operators, Lagrangian-Eulerian existence theorems, and critical dissipative active scalars. Annotation ©2019 Ringgold, Inc., Portland, OR (protoview.com)