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The large increase in size and complexity experienced by cellular networks in recent years has led to a new paradigm known as heterogeneous networks, or HetNets. In this context, networks with different cell sizes, radio access technologies, or carrier frequencies can be deployed in the same environment. As the coverage area of each of these networks is typically overlapped, operators have some degree of freedom to modify user distributions across the networks (i.e., traffic steering) in order to improve network performance. This article introduces different mechanisms of traffic steering in HetNets, clarifying the specific goals that operators can set and focusing on those techniques that adjust mobility parameters, which are typically more attractive to achieve these goals. In addition, some challenging issues arising from particular HetNet deployments are discussed and illustrated by example use cases, which are applicable to an early stage of LTE deployment. Finally, a fuzzy-logic-based algorithm that optimizes network parameters for traffic steering is proposed.