In this issue of Problems and Techniques, we feature articles on the formation of structure in flows involving immiscible liquids and techniques for solving nonlinear knapsack problems. Free surface and interface problems introduce the added complication of having to determine the position of unknown surfaces into a typically complex scientific computation problem. When surfaces develop intricate patterns and change topology as solutions evolve, numerical approaches must simultaneously maintain accuracy and control computational cost. In the first article, Jie Li and Yuriko Renardy describe a volume of fluid (VOF) technique for addressing instabilities in slow flows involving two immiscible, incompressible, viscous liquids subject to shearing. After describing their variant of the VOF method, Li and Renardy use it to analyze the simple shearing of two fluids in parallel flow (two-layer Couette flow), the flow of a core of fluid surrounded by a second fluid in a pipe (core-annular flow), and droplet breakup. Their results explain how the interface forms "fingers" of one fluid into the other, "bamboo waves" between the fluids, and the rupture of larger drops into trains of smaller ones. Experimental observations nicely confirm and complement the calculations. The second article, by A. Melman and G. Rabinowitz, describes a globally convergent iterative procedure for a class of nonlinear knapsack problems. As I learned, a knapsack problem involves a maximization of a concave function subject to a linear constraint. The optimization presented by Melman and Rabinowitz is motivated chemical processing where periodic servicing is necessary to remove waste by-products and to add raw materials. The operations research problem is to determine the optimal service schedule. Using quadratically convergent Newton-like iterations, the authors construct a fast solution procedure and show its global convergence for a subclass of knapsack problems. Complications arise because the function whose roots they seek is singular and has only an implicit representation. Once again, I'm pleased to thank our authors for their interesting and informative contributions.