Whilst Spoken Language Dialogue Systems (SLDSs) technology has made good progress in recent years, the issue of SLDS usability is still lagging behind both theoretically and in actual SLDS development and evaluation. However, as more products reach the market and competition intensifies, there is growing recognition of the importance of systematically understanding the factors which must be taken into account in order to optimise SLDS usability. Ideally, this understanding should be comprehensive (i.e. include all major human factors perspectives on SLDSs), and exhaustive (i.e. describe each perspective as it pertains to the detailed development and evaluation of any possible SLDS). This paper addresses the requirement of comprehensiveness by decomposing the complex space of SLDS usability best practice into eleven issues which should be considered by developers during specification, design, development and evaluation. The discussion of each issue is aimed to support the developer in building SLDSs which are likely to generate user satisfaction, which are perceived to be easy to understand and control, and which enable smooth user- system interaction. Based on the best practice issues discussed, criteria for evaluating SLDS usability are proposed. Several limits to our current understanding of SLDS usability are highlighted.