This paper focuses on soundscape planning, or acoustic design, in the planning and management of open space in both urban and non-urban areas. It is based on notions, promoted over several decades, that the acoustic aspects of open space can, and should be, subject to design in the same way as are the visual dimensions. The current paradigm for the management of the outdoor acoustic environment is noise control and soundscape planning needs to adopt quite different practices from noise control with respect to acoustic criteria and measurement. The paper explores the specification of acoustic objectives for outdoor soundscapes and the translation of these objectives into acoustic criteria that are amenable to measurement and prediction as part of the design process. Such objectives, termed Proposed Acoustic Environments, focus on the information content in sounds in a particular space and, only indirectly, on characteristics such as level or loudness. Outdoor acoustic design is mostly concerned with avoiding, or achieving, the masking of one set of information in the acoustic signal with other sets of information in the same signal. These are critical methodological issues if soundscape planning is to move from being a good idea to common practice.