Underwriting and investing
The business model can be reduced to a simple equation: Profit = earned premium + investment income - incurred loss - underwriting expenses.
Insurers make money in two ways: (1) through underwriting, the process by which insurers select the risks to insure and decide how much in premiums to charge for accepting those risks and (2) by investing the premiums they collect from insured parties.
The most complicated aspect of the insurance business is the underwriting of policies. Using a wide assortment of data, insurers predict the likelihood that a claim will be made against their policies and price products accordingly. To this end, insurers use actuarial science to quantify the risks they are willing to assume and the premium they will charge to assume them. Data is analyzed to fairly accurately project the rate of future claims based on a given risk. Actuarial science uses statistics and probability to analyze the risks associated with the range of perils covered, and these scientific principles are used to determine an insurer's overall exposure. Upon termination of a given policy, the amount of premium collected and the investment gains thereon minus the amount paid out in claims is the insurer's underwriting profit on that policy. Of course, from the insurer's perspective, some policies are "winners" (i.e., the insurer pays out less in claims and expenses than it receives in premiums and investment income) and some are "losers" (i.e., the insurer pays out more in claims and expenses than it receives in premiums and investment income); insurance companies essentially use actuarial science to attempt to underwrite enough "winning" policies to pay out on the "losers" while still maintaining profitability.
An insurer's underwriting performance is measured in its combined ratio. The loss ratio (incurred losses and loss-adjustment expenses divided by net earned premium) is added to the expense ratio (underwriting expenses divided by net premium written) to determine the company's combined ratio. The combined ratio is a reflection of the company's overall underwriting profitability. A combined ratio of less than 100 percent indicates underwriting profitability, while anything over 100 indicates an underwriting loss.
Insurance companies also earn investment profits on “float”. “Float” or available reserve is the amount of money, at hand at any given moment, that an insurer has collected in insurance premiums but has not been paid out in claims. Insurers start investing insurance premiums as soon as they are collected and continue to earn interest on them until claims are paid out. The Association of British Insurers (gathering 400 insurance companies and 94% of UK insurance services) has almost 20% of the investments in the London Stock Exchange.
In the United States, the underwriting loss of property and casualty insurance companies was $142.3 billion in the five years ending 2003. But overall profit for the same period was $68.4 billion, as the result of float. Some insurance industry insiders, most notably Hank Greenberg, do not believe that it is forever possible to sustain a profit from float without an underwriting profit as well, but this opinion is not universally held. Naturally, the “float” method is difficult to carry out in an economically depressed period. Bear markets do cause insurers to shift away from investments and to toughen up their underwriting standards. So a poor economy generally means high insurance premiums. This tendency to swing between profitable and unprofitable periods over time is commonly known as the "underwriting" or insurance cycle. 
Property and casualty insurers currently make the most money from their auto insurance line of business. Generally better statistics are available on auto losses and underwriting on this line of business has benefited greatly from advances in computing. Additionally, property losses in the United States, due to unpredictable natural catastrophes, have exacerbated this trend.
Claims and loss handling is the materialized utility of insurance; it is the actual "product" paid for, though one hopes it will never need to be used. Claims may be filed by insureds directly with the insurer or through brokers or agents. The insurer may require that the claim be filed on its own proprietary forms, or may accept claims on a standard industry form such as those produced by ACORD.
Insurance company claim departments employ a large number of claims adjusters supported by a staff of records management and data entry clerks. Incoming claims are classified based on severity and are assigned to adjusters whose settlement authority varies with their knowledge and experience. The adjuster undertakes a thorough investigation of each claim, usually in close cooperation with the insured, determines its reasonable monetary value, and authorizes payment. Adjusting liability insurance claims is particularly difficult because there is a third party involved (the plaintiff who is suing the insured) who is under no contractual obligation to cooperate with the insurer and in fact may regard the insurer as a deep pocket. The adjuster must obtain legal counsel for the insured (either inside "house" counsel or outside "panel" counsel), monitor litigation that may take years to complete, and appear in person or over the telephone with settlement authority at a mandatory settlement conference when requested by the judge.
In managing the claims handling function, insurers seek to balance the elements of customer satisfaction, administrative handling expenses, and claims overpayment leakages. As part of this balancing act, fraudulent insurance practices are a major business risk that must be managed and overcome. Disputes between insurers and insureds over the validity of claims or claims handling practices occasionally escalate into litigation; see insurance bad faith.