Knowledge about landscapes is highly complex and it is important to clarify how that complexity is reflected in the knowledge claims that feed into a particular planning process. Thus, this paper addresses critical issues and challenges regarding the formation of knowledge about landscapes in spatial planning contexts, based on published landscape research and planning theory. The analysis is rooted in planning theorists’ discussion of various types of knowledge claims involved in spatial planning practices. Thinking in terms of knowledge, and discussing both the character of knowledge production and types of knowledge claims that will be included, is a useful approach for choosing and developing assessment methods. To aid such approaches, two key aspects of formation of knowledge about landscapes are addressed here. One is the trans-disciplinary challenge of capturing landscapes as a whole. The other is the normative element of knowledge pertaining to landscapes, including diverging moral and ethical perspectives.