Mental health clinical practice guidelines have proliferated, but there is little evidence regarding the degree to which they are actually implemented in clinical practice. The goal of this study was to locate and review all peer-reviewed reports published through 2000 that provide quantitative information on rates of adherence to specific mental health guidelines. A literature search yielded 41 pertinent studies. These studies were of three types: 26 were cross-sectional investigations performed after the release of guidelines, six were conducted before and after release of guidelines without any specific intervention (pre/post), and nine involved a controlled trial of a specific intervention. Only 37% were conducted in the mental health specialty sector. Adequate adherence was found in 27% of the cross-sectional and pre/post studies but in 67% of the controlled trials. Successful interventions tended to be complex, involving system redesign or additional resources. Only six of 13 investigations (46%) that also measured patient outcome found that better outcome was associated with greater guideline adherence. Several studies showed that after cessation of interventions, adherence rates returned to preintervention levels. Thus, evidence indicates that guideline adherence is not high without specific intervention, but that certain interventions (typically multifaceted and resource-intensive ones) improve adherence. However, the public health challenge is to design and implement interventions that are sustainable in general clinical practice.