CBT is increasingly offered through the Internet. Internet-based interventions may have several advantages over other more traditional forms of delivery. They may save therapist time, reduce waiting-lists, allow patients to work at their own pace, abolish the need to schedule appointments with a therapist, save traveling time, reduce the stigma of going to a psychologist or therapist, and facilitate help for the hard-of-hearing as self-help treatments typically work with visual rather than auditory information (Marks et al. 2007). Furthermore, Internet-delivered self-help may be programmed to enhance patients’ motivation by presenting a wide range of attractive audiovisual information with voices giving instructions in whichever gender, age, accent, language and perhaps game format the client prefers. It can also quickly and automatically report patient progress and self-ratings.

Internet-based interventions may reach populations with health problems, who can not be reached with other more traditional forms of treatments. For example, a considerable proportion of the patients with mental disorders are not reached with traditional forms of treatment (Bijl and Ravelli 2000) because of the stigma associated with mental disorder, prejudices about therapists, lack of willingness to talk to a stranger about personal problems, or because of physical obstacles like walking problems or long distances. For patients with somatic conditions there may be barriers to seek psychological treatment. Internet-guided interventions may reach a segment of this population who cannot be reached through traditional interventions.

Internet-based psychological interventions for many health problems are commonly based on CBT techniques. One reason is that the effects of CBT have been shown in numerous trials. Another reason why CBT is often used in internet-interventions is that these techniques lend themselves to be operationalized in text. CBT interventions can very well be converted into a structured format, with psychoeducation, homework assignments and registration exercises presented via web pages (Ritterband et al. 2006).

Delivering CBT through the Internet does not, however, only have advantages. An online programme may not be suitable for technophobic patients and illiterates, nor can it answer all the possible questions users may ask; it can not detect subtle nonverbal and verbal clues to clients’ misunderstandings; it may encourage clients to cherry-pick from a range of homework options presented; and not all clients find communicating via computers acceptable (Marks et al. 2007). However, subtle text nuances may be detected and somewhat surprisingly, Internet interventions has been found to generate good working alliance between the patient and the therapist (Knaevelsrud and Maercker 2006).

Furthermore, it is not yet very clear whether CBT interventions which have been proven to be effective when delivered in traditional format, are also effective when delivered through the internet. Recent studies, however, indicate that this might be the case for at least some patients and some conditions (Carlbring et al. 2005).

Whether or not an internet-based CBT is effective should be examined in randomized controlled trials, and can obviously not be based on the effects of traditionally delivered CBT (e.g., individual or group format). Since the Internet has become available to the broad public in many Western countries, several trials have examined the effects of cognitive behavioral interventions in randomized trials. In a recent meta-analysis, we examined the effects of internet-based treatments for depression and anxiety disorders, and found that these interventions had large effects (Cohen’s d = 1.00) compared to control conditions, when some kind of guidance was given to the patients receiving the treatment (Spek et al. 2007). This latter observation was also confirmed in a recent review in which a correlation of rho = .75, P < .005 was obtained between amount of contact spent with clients and the effect size (Palmqvist et al. 2007). These large effect sizes suggest that Internet-administered CBT is as effective as face-to-face CBT, and that the format in which CBT is delivered may not be related to the effect sizes found. It is not known, however, whether Internet-administered CBT is also equally effective when other health problems are targeted.

Although several health problems were targeted in these Internet-based studies, there are gaps in the literature in terms of treatments for health problems which have been found to improve by means of CBT. For example, several studies have examined the effects of CBT for chronic fatigue syndrome (Knoop et al. 2007), fibromyalgia (Garcia et al. 2006), incontinence (Garley and Unwin 2006), or multiple sclerosis (Thomas et al. 2006), but these have not yet been transformed into a web based intervention, although trials found positive effects of face-to-face CBT for these problems. Because the promising results of earlier studies, and because of the benefits of Internet-based interventions, we can expect development of new programs for these conditions in the future.

Our review does not cover the whole field of internet interventions. While we focused on CBT for existing health problems, several other studies have examined CBT for mental health problems (Spek et al. 2007), on internet-based preventive interventions aimed at a healthy lifestyle (weight loss, smoking, exercise; e.g., Swartz et al. 2006; Muñoz et al. 2006; Oenema et al. 2001; Tate et al. 2006), and interventions using non-CBT methods (McMahon et al. 2005; Edwards et al. 2006; Gray et al. 2000). However, as was shown in this review, research on CBT interventions has been growing fast in the past few years. Because CBT interventions are very well suited to be used through the internet, it can be expected that research in this area will continue to grow further in the next years.

The included studies do not yet allow us to draw definite conclusions about whether CBT through the Internet are as effective as face-to-face interventions. For most health problems we found only one study examining the effects of an Internet-based CBT study. In fact, it was only for pain and headache did we find more than one studies. However, the effects found for Internet-based interventions aimed at pain are comparable to the effects found for face-to-face treatments for pain (Morley et al. 1999), and the same is most likely true for the Internet-interventions aimed at headache (Bogaards and ter Kuile 1994). The other interventions also found some effects, although some effects were stronger than others. It does seem clear, however, that Internet-based CBT can have significant effects on some of the health problems described in this review. For at least one of the conditions—tinnitus—an effectiveness study has been published showing better results then the first controlled efficacy trial (Kaldo-Sandström et al. 2004).

It has been suggested self-help interventions be used as one of the first steps in stepped-care programs (Scogin et al. 2003). Perhaps Internet-based interventions which are used in healthcare settings should also be placed within these stepped-care frameworks. In these cases, additional care is available if the Internet-based intervention does not reduce the problem of a patient sufficiently. On the other hand, Internet interventions can develop as well, and might at least for some patients be more suitable than face to face CBT. As many health conditions such as chronic pain and cancer require a multidisciplinary team approach for optimal treatment, we assume that future Internet interventions will take advantage of this possibility.

There is no consensus yet among researchers about the way CBT should be presented on the Internet, although standards are emerging. Most interventions used a guided self-help format in which the treatment protocol is presented on the Internet and the patient works it through more or less independently. The patients are supported by brief contacts with therapists through e-mail or telephone. However, other studies use a more traditional format in that the patients go online at the same time as the therapist and have a more or less regular treatment session. Group treatments can also be delivered in such a way.

Another difference between interventions concerns the additional elements on the Internet, apart from CBT. Some interventions have combined the cognitive behavioral interventions with other components, such as psychoeducation, films and texts to read, and a forum for users of the website. Other interventions do not provide such extras.

Our review showed up several other important limitations of the current research in this area. First, most studies used waiting list control groups, and only very few used a care-as-usual or another control group. Subjects in waiting list control groups probably do not take constructive action to reduce their problems themselves during the waiting period, because they are expecting professional help in the future. This may result in an overestimation of the effects of an intervention, because there may be less spontaneous recovery.

Second, most studies recruited participants through the community and through other websites. This is not a problem for interventions that target the general population. But when such an intervention is effective this does not automatically mean that it is also effective in clinical settings. Subjects who are responding to community recruitment are probably very motivated which may improve their results compared to subjects who receive treatment.

Third, none of the twelve identified trials compared Internet-based treatments to face-to-face or other treatments. This is, however, an important issue, because only direct comparisons can give evidence about the comparative effects of Internet-based treatments compared to more traditional treatments and the type of patients who can benefit from it.

Fourth, most studies were aimed at adults. Only two studies were aimed at children and adolescents, while these groups are probably the most familiar with the Internet. None of the studies were specifically aimed at older adults, while they suffer most from health conditions.

Future research should focus on these limitations of current research. More studies are needed with care-as-usual or other control groups, clinical recruitment strategies, comparisons with face-to-face treatments, and children or older adults as target populations. More research is also needed to examine how CBT should be presented on the Internet, and to examine reasons and solutions to the relatively high drop-out rates in several studies. Finally, it is also important to study how Internet-administered CBT can be integrated in stepped-care models of care.

This review has several limitations. First, the number of included studies is still very small. And the number of studies examining specific health problems is too small to integrate the results of these studies statistically into a meta-analysis. Second, the quality of the included studies is not optimal. Third, the drop-out rates reported are high in some studies. This is a concern for this type of intervention, as patients can very easily withdraw from the intervention. Remarkably, the studies in which more traditional therapies (live sessions with therapists) are delivered through the Internet have the lowest drop-out rates.