The objective of this study is to update evidence-based best practice guidelines for psychological evaluation and treatment of weight loss surgery (WLS) patients. We performed a systematic search of English-language literature on WLS and mental health, quality of life, and behavior modification published between April 2004 and May 2007 in MEDLINE and the Cochrane Library. Key words were used to narrow the search for a selective review of abstracts, retrieval of full articles, and grading of evidence according to systems used in established evidence-based models. Our literature search identified 17 articles of interest; 13 of the most relevant were reviewed in detail. From these, we developed evidence-based best practice recommendations on the psychological assessment and treatment of WLS patients. Regular updates of evidence-based recommendations for best practices in psychological care are required to address the impact of mental health on short- and long-term outcomes after WLS. Key factors in patient safety include comprehensive preoperative evaluation, use of appropriate and reliable evaluation instruments, and the development of short- and long-term treatment plans.