The rates of overweight and obesity are rising in Canada and worldwide, and there is a need for effective methods for weight loss and weight maintenance to empower individuals to make changes. The purpose of this systematic review was to examine the evidence available for successful diet strategies for weight loss and weight maintenance among adults. A search was conducted of the following databases: CAB Abstracts, Central Register of Controlled Trials, EMBASE, MEDLINE, Food Science and Technology Abstracts, and Web of Knowledge. The studies investigated had participants who were overweight or obese and between 18 and 65 years of age. A successful study was defined as one that reported an intervention that created ≥5% weight loss from baseline and a maintenance phase during which the ≥5% weight loss was maintained from baseline to 12 months. After exclusions, the search resulted in 67 papers. Overall, for significant safe weight loss, an energy deficit was required, which was commonly achieved by reduced fat intake. Increased dietary fibre was also a component of 21% of successful interventions. Physical activity was included in 88% of successful interventions, and behaviour training such as self-monitoring was part of 92% of successful interventions. The same combination of energy and fat restriction, regular physical activity, and behavioural strategies was also required for successful weight maintenance. This review confirmed previous knowledge about weight loss and weight maintenance in adults. A comprehensive approach, including reduced dietary intake, regular physical activity, and behavioural strategies, is warranted and is supported by the research evidence.