The intent of this study is to present effectual methodologies concerning single-phase and two-phase mixing problems which may conveniently be utilized by a design or developmental engineer. In particular, for single phase, turbulent, compressible coaxial fuel and air mixing problems, an index is presented which provides a sensitive indicator for determination of the degree of mixing in a given mixing chamber. Working graphs for several hydrocarbon fuel and air stream combinations are used to demonstrate the utility of the index. In addition, the index is shown to be of value for studying twophase mixing such as occurring in automotive carburetors.This paper further demonstrates the utility of the hydraulic analogy for studying complex mixing problems such as commonly occurring in automotive carburetors and manifolds, gas turbine combustors, and simple mixing chambers. Use of the hydraulic analogy technique is shown to be a fast, inexpensive method which provides a pictorial representation of flow fields. Interpretation of the flow field stream line patterns brings about an improved understanding of conditions producing pressure drops, local flow field instabilities which are possible points of noise generation, improved flow field guidance through effective use of bounding walls or deflectors, and/or of conditions affecting local heat transfer rates.