Diver training is the set of processes through which a person learns the necessary and desirable skills to safely dive underwater within the scope of the diver training standard relevant to the specific training programme. Most diver training follows procedures and schedules laid down in the associated training standard, in a formal training programme, and includes relevant foundational knowledge of the underlying theory, including some basic physics, physiology and environmental information, practical skills training in the selection and safe use of the associated equipment in the specified underwater environment, and assessment of the required skills and knowledge deemed necessary by the certification agency to allow the newly certified diver to dive within the specified range of conditions at an acceptable level of risk. Recognition of prior learning is allowed in some training standards.

Recreational diver training has historically followed two philosophies, based on the business structure of the training agencies. The not-for profit agencies tend to focus on developing the diver's competence in relatively fewer stages, and provide more content over a longer programme, than the for-profit agencies, which maximise profit and customer convenience by providing a larger number of shorter courses with less content and fewer skills per course. The more advanced skills and knowledge, including courses focusing on key diving skills like good buoyancy control and trim, and environmental awareness, are available by both routes, but a large number of divers never progress beyond the entry level certification, and only dive on vacation, a system by which skills are more likely to deteriorate than improve due to long periods of inactivity. This may be mitigated by refresher courses, which tend to target skills particularly important in the specific region, and may focus on low impact diving skills, to protect the environment that the service provider relies on for their economic survival.

Diver training is closely associated with diver certification or registration, the process of application for, and issue of, formal recognition of competence by a certification agency or registration authority. The training generally follows a programme authorised by the agency, and competence assessment follows the relevant diver training standard.

Training in work skills specific to the underwater environment may be included in diver training programmes, but is also often provided independently, either as job training for a specific operation, or as generic training by specialists in the fields. Professional divers will also learn about legislative restrictions and occupational health and safety relating to diving work.

Sufficient understanding of the hazards associated with diving activities is necessary for the diver to be competent to reasonably assess and accept the risk of a planned dive. The professional diver can to some extent rely on the diving supervisor, who is appointed to manage the risk of a diving operation, and a diver in training can expect the instructor to adequately assess risk on training dives. Certification agencies minimise their responsibility by limiting the conditions in which the diver is considered competent.

Both recreational and professional diving occur in the underwater environment, which presents the same basic hazards to both groups. The professional diver has a job to do in this environment, which may expose them to additional hazards associated with the work. The recreational diver usually has no legal duty of care to other divers. The duty of care to a recreational dive buddy is poorly defined and with some exceptions, the diver is not legally obliged to dive with a buddy unless they choose to. The professional diver is a member of a diving team and has legally defined duty of care to other members of the team.

Recreational diver training prepares the diver for low stress diving in an environment similar to the one they were trained in, using equipment similar to that used in training. Professional diver training prepares the diver for a working environment in which the diver is expected to work as a member of a team, and be involved in organisation, planning, setting up of the infrastructure, selection and maintenance of the diving and work related equipment, where conditions may be less than ideal, and there may be time constraints. The professional diver must be able to make a realistic and informed decision on acceptability of risk.

Recreational diver training is regulated within the recreational diver training industry, within each certification agency, and tends to be kept to a minimum to keep costs down in a competitive environment, and because of customer pressure to minimise the effort involved. Consequently, the entry level certification for most recreational divers advises them to dive only in conditions similar to those in which they were trained, and to a maximum depth of 18 m, with no decompression obligation. This also encourages the diver to attend further training if they wish to achieve more than minimum competence. The training is short, convenient, and minimal. The cost is low in absolute terms, but accumulates with further training, for those few who undertake it. Professional diver training, and the associated assessment and certification of competence, are usually based on occupational health and safety legislation, and also covered by the employer's duty of care. The training standards are usually aligned with internationally recognised standards, and are expected to follow quality assurance procedures. The professional diver with entry level qualification is expected to perform duties as working diver, diver's attendant, and standby (rescue) diver, and must be competent and fit to perform all these tasks, as well as the basic skills of staying alive underwater and not getting injured, and the work involved in setting up the site and demobilisation after the job.