Solo diving is considered technical diving, and for excellent reasons. Most certification agencies discourage solo diving; rather they teach the buddy diving safety system. Some divers, such as instructors, are inadvertently solo divers because they dive with buddies who may not be capable of rescuing them.
Safe solo divers must be self sufficient, well trained, prepared and practiced. They should have a completely redundant set of all life support equipment. In addition, the responsible solo divers adhere to a very conservative dive profile, both in depth and level of difficulty. Unlike the buddy system, which encourages divers to rely on others in the event of an emergency, solo diving encourages divers prepare themselves to overcome emergencies by their own means. The divers who engage in solo diving are typically those who are experienced and equipped enough to handle problems themselves. Lack of trust for other divers is often cited, ie; the buddy is more likely to experience problems than diver.
While there are potential hazards involved with solo diving, most of these can be planned for and mitigated by the proper use of redundancy in equipment. In technical diving, where redundancy is standard, self-sufficiency is taught more strongly. In many situations if a diver has a problem other divers may not have sufficient gas to complete the dive for both. This is especially true of cave diving where stressful situations can vastly increase gas consumption and there is no option to skip decompression. A solo diver would need a second, independent source of air, a complete second set of regulators (both first and second stage regulators, and optionally an air gauge for his/her alternate source of air). As with all scuba equipment, the diver must be intimately familiar with this configuration and have the ability to access any of the equipment easily if it should be needed.
There has been much controversy over the relative safety and merits of solo diving. Very little statistics currently exist regarding the impact of solo diving on safety. This is primarily because it is a relatively new area of discussion in the diving community, as well as the fact that it is and has been frowned upon in general by most Dive Certification Agencies. Until such statistics can be gathered and studied, it is safe to say that it is an area that will remain a focus of some controversy.