Ecosystem services refer to the benefits or outputs that people derive from ecosystems. Following the publication of the UK National Ecosystem Assessment there has been a growing interest in assessing the flows of such services and valuing the contribution they make to human well-being. This Research Report draws upon recent evidence (years 2001 to 2012) from the behavioural economics literature to examine how cognitive factors influencing people’s choices and preferences can affect the values that they place upon ecosystem services and upon ecosystem sustainability. The Report shows that there can be a wide variation in the values placed on particular ecosystem services due to a range of factors. For example, the ability of individuals to process information can result in eight times higher variance in respondent values when more complex formats are used. The Report covers methods used to mitigate these effects and highlights where addressing research gaps on how people value ecosystem services could contribute to ecosystem sustainability.