Once a duty of care has been established not only should the defendant owe a duty of care, but also must be in breach of that duty. Therefore the defendant should have failed to come up to the standard of care required by law for fulfilment of duty. What is the standard of care and has the standard been breached are two questions that will arise to establish a breach of that duty. Standard of care required in negligence law typically relates to a person’s conduct, rather than a person’s state of mind.
The basic rule is that the defendant must conform to the standard of care expected of a reasonable person. The so-called reasonable person in the law of negligence is a creation of legal fiction. This legal fiction steps into the shoes of the defendant and such a “person” is really an ideal, focusing on how a typical person, with ordinary prudence, would act in certain circumstances. The actual defendant maybe stupider or more ignorant or maybe cleverer or more knowledgeable but is still judged by this abstract impersonal standard.
The actual knowledge and experience of the defendant will also be taken into account. It is therefore a question of foreseeabilty rather than probability. Foreseeabilty can only be discovered as mentioned previously through actual knowledge and experience. Whereas probability will not depend on those factors.  For instance one may consider a defendant working on a loading dock and tossing large bags of grain onto a truck. During the process, defendant notices two children playing near the truck. The defendant throws a bag towards the truck and unintentionally strikes one child. In this instance, a jury would take into account the defendant’s actual knowledge that children were playing in the area when the jury determines whether the defendant acted reasonably under the circumstances. One must note, however, that the defendant would be liable for negligence only if the defendant owed a duty to the child.
According to the dictum of Alderson B, the objective standard is defined as –
“Negligence is the omission to do something which a reasonable man guided upon those considerations which ordinarily regulate the conduct of human affairs would do,or something which a prudent and reasonable man wouldn’t do.” 
In Glasgow Corporation v Muir the House of Lords stated that the standard of foresight of the reasonable man is an impersonal test independent of the idiosyncrasies of the particular Defendant. Therefore that it is an objective test.
Further the relationship between the Plaintiff and Defendant may also lead the courts to modify the standard required; Goldman v Hargrave.”  This was a case where there was a failure to extinguish a fire started by natural causes. It took into account what was reasonable to expect in his individual circumstances.
It is left to the judge to decide what in the circumstances of a reasonable man would have had in contemplation and thereby room for diversity of view. Therefore the outcome maybe unpredictable even in the tiny majority of cases.
Where the Defendant is in breach of duty and thus negligent; if the plaintiff is exposed to an unreasonable risk of harm, the court must weigh a number of factors in the balance. .As the danger increase so must the precaution. There must be a balance between the magnitude of the risk and the burden to the defendant in doing what the defendant should or should not have done; The magnitude of the risk includes the likely hood of harm, the gravity of harm. Also social utility of the activity and the cost and practicability of precautionary measures to minimize or eliminate the risk and make a value judgment as to what a reasonable man would’ve done in the circumstances.
Under magnitude of risk; likely hood of harm; In Bolton v Stone; the ball hit from the cricket ground and hit the Plaintiff. This happens once every five years and therefore it was held that there was no negligence. Reasonable man do in fact take into account the degree of risk and do not act upon a bare possibility as they would if the risk were more substantial.  In terms of Gravity of harm the greater the possibility to harm the plaintiff the greater the chance of liability; Paris v Stephey; which involved a risk of an eye injury, the duty of care is owed to the plaintiff himself and if he suffers from some disability, the disability must be taken into account as long as it is or should be known to the defendant.
Asquith L.J stated that it is necessary to balance the risk against the consequences of not taking it. Therefore in terms of Social utility would be where the validity to society of the defendants activities are examined; Watt v Hertfortshire CC it was held that the fire authorities had not been negligent for the risk involved to W was not so great as to prohibit the attempt to save life. The commercial end to make profit is very different from the human end. 
Finally in the case where cost and practicability of precautionary measures to minimize or eliminate the risk; in the case of Latimer, a factory floor became slippery after a flood. It was held that the defendant had done all they could to prevent the injury n thereby not negligent. The greater the risk, the less receptive a court is likely to be to a defence based simply upon cost in terms of money of the required precautions.
Most difficult cases involve defendants with special skills or qualifications. Where the defendant is exercising a special skill or belongs to a particular profession the defendant has to conform to the standards of a reasonably competent member of that profession.
If a driver collapses and the passenger tries to bring the vehicle to halt, it would not amount to negligence. However in cases where a person is in exercise of a particular skill, law expects him to show the amount of competence associated with a proper discharge of the duties of that profession. The Roman term “Imeritia Culpae adnumeratur”  comes into play similar to that of English Law.
In Phillips v Whiteley; the plaintiff arranged for her ears to be pierced by a jeweller and as a result infected her ears. It was held that the jeweller was not liable as they had never claimed to reach the standard of a surgeon.
In terms of a special skill the hallmark case of Bolam v Friern Hospital established a test where a man need not possess the highest expert skills. The issue was whether a doctor failing to prescribe a relevant drug before treatment was negligent. It was held that the Defendants were not liable. The question was whether the standard of an ordinary skilled man exercised and professed to have that special skill. There is no uniformity as to what is proper. Subsequently it was adopted in Roe v Minister of Health and the Bolam test applied to all medical practitioners, doctors, surgeons etc. In the case of Roe it was held that the hospital was not liable because it was not reasonably foreseeable at that time.
In the case of Wilsher v Essex the Cout of Appeal rejected the argument that a junior inexperienced Doctor owes a lesser standard of care than a more experienced doctor. Mustill LJ stated that a standard of care which patient is entitled to demand would vary according to the chance of recruiting and rostering. Therefore a general practitioner can be expected to have the expertise of a specialist but should when necessary take appropriate specialist advice.
In the case of Lawyers the test to be applied is what is expected of a reasonably competent council of the appellants seniority and experience; Moy v Pettman Smith. Courts will be able to rely upon its own knowledge and experience. However in terms of the medical profession the courts should not attempt to put itself into the shoes of a surgeon; Sidaway v Bethlem Royal Hospital.
The relationship between the plaintiff and defendant might also lead the courts to modify the content of the Defendants duty where the Plaintiff submitted himself to treatment by someone whom he knew of limited competence. In Nettleship v Weston,  a learner driver should come up to the standards of an ordinary competent driver. No reasonable man handles a stick of dynamite and a walking stick in the same way. 
Children may be liable in negligence and are judged by what might be expected of a reasonable child of the defendants age; Mullin v Richards; Parents ,may also be liable for instance “Would a reasonable, careful parent leave his child out of his or her sight for a second in that vicinity?”  Thus it would be a breach of that standard.
When the courts find a clearly established practice the burden of proof is a heavy one. If not, it reverses it and requires him to justify his conduct.
In conclusion therefore the actual defendant is to be compared with a reasonable man in the same circumstances and whether the standard has been breached, is a question of fact. The court looks at the actions of the defendant to see if his actions measure up to the standard. If they do not, the Defendant is in breach.