The Knapsack Problem is among the most well-known and widely studied optimization problems. Given a set of items, each item with an associated weight, the problem asks for a subset of items with a total weight no larger than an available capacity and which maximizes a corresponding measure of profit. The problem appears in distinct versions and also in accompanying variants to these versions, each version being defined by a different type of objective function. Every variant, in turn, imposes some additional requirements on item selection. In this chapter we review a subset of these variants, with a focus on recent papers that bring interesting new applications for the problem