ed the amplification options available to both children and adults. The complexity of these systems has resulted in a need for new and efficient fitting procedures and protocols. For adults with hearing loss, the fitting process often is supplemented with a variety of subjective measures such as judgments of loudness, clarity, or intelligibility. For obvious reasons, these types of measures cannot be obtained reliably from infants and young children. A second fitting approach is to use a prescriptive algorithm that is based on each individual’s audiometric thresholds. Numerous threshold-based procedures have been developed for use with linear hearing aids (Byrne and Dillon 1986; Cox 1988; McCandless and Lyregaard 1983; Seewald 1992) and, more recently, nonlinear hearing aids (Cornelisse, Seewald, and Jamieson 1995; Killion 1995; Dillion et al. 1998). The majority of these procedures, however, were developed using data from adults with hearing loss. Only the desired sensation level (DSL) procedure (versions 3.1 and 4.0) was designed to specifically account for the many differences between young children and adults (Cornelisse, Seewald, and Jamieson 1995; Seewald 1992). The underlying goal of this procedure is to provide an amplified signal that is both audible and comfortable across as broad a frequency range as possible. Using this procedure, all audiometric and electroacoustic data are transformed to an equivalent ear canal sound pressure level (SPL) to facilitate comparisons between audiometric results and hearing aid data. The known age-related changes in external ear canal characteristics (Bentler 1991; Kruger 1987; Kruger and Ruben 1987) and real-ear-to-coupler differences (Feigin et al. 1989; Nelson Barlow et al. 1988) are taken into consideration in the development of target gain and maximum output values. In addition, the speech spectrum used to derive target values takes into account the child’s need for auditory self-monitoring of speech (Cornelisse, Gagné, and Seewald 1991a). The underlying assumption is that speech must be audible in order to optimize the use of residual hearing. In this chapter, a variety of issues related to the definition of audibility will be discussed in relation to the hearing aid fitting process.