One in every nine women in the UK will develop breast cancer at some point in her life - more than 41,000 cases are diagnosed each year.
It has become the most common cancer in the UK, and is the leading cause of death for women aged 34 to 54.
More rarely, men can also develop this cancer.
Despite recent improvements in the mortality rates, due to better treatments and earlier detection, the UK still has one of the highest mortality rates in the world.
But research is beginning to reap dividends in new ways of understanding how breast cancer cells work.
Professor Charles Coombes, who is director of Cancer Research UK's research laboratory at Imperial College London, says there is cause for optimism.
"The more we understand about how these cells behave, the more likely we are to understand what happens with breast cancer.
"That revolution is ongoing. We are going, I think, to be looking at improved results over the next 20 or 30 years."
The most common way that a potential problem is detected is when physical changes are noticed in the breasts.
Regular breast screening may also highlight changes in the breast.
The key is for the woman to know what "normal" is - then changes can be noticed.
Examples of the kind of things to look out for include:
* a change in outline, shape or size of the breast
* puckering or dimpling of the skin
* any lump or thickening in the breast or armpit
* any flaking skin or discharge from the nipple
* unusual pain or discomfort
Any changes should be reported to a doctor - although most will turn out not to be cancerous.
Many lumps will be picked up with mammograms - x-rays of the breast taken every few years as part of the NHS national screening programme.
If a lump is found, techniques used to investigate it include ultrasound and "fine needle aspiration", which will show whether the area is a solid lump or is a cyst.
A biopsy may also be carried out, so that a sample of the lump can be examined in a laboratory.
The precise reasons why a woman develops breast cancer are still unknown, but are thought to be a combination of genetic, environmental and lifestyle factors.
Scientists have identified two genes which are more likely to be defective in a breast cancer patient than someone without breast cancer.
These genes are also blamed for some other cancers.
However, even the two mutated genes are thought only to be responsible for approximately 5% to 10% of breast cancer cases.
Hormones seem to have an important role in breast cancer. Research has shown a link between levels of the female sex hormone, oestrogen, and the risk of developing breast cancer.
Women who take certain types of hormone replacement therapy are at higher risk of breast cancer.
Women who have their first child later in life also appear to be at higher risk of developing breast cancer.
If cancer is confirmed, then there are variety of treatments available, depending on the size of type of the tumour, and whether doctors believe it has or could have spread.
Most women with breast cancer do not need to have a breast removed.
The bigger the tumour relative to the size of the breast, the more likely that mastectomy will be recommended.
In a procedure known as a "lumpectomy", just the cancerous lump is removed.
After both kinds of operation, radiotherapy may be given to reduce the chance that the cancer will return.
If the tumour is very large, treatment may be given to reduce the size of the tumour before the operation takes place.
In most cases, the surgeon also removes lymph nodes under the arms to find out if the cancer cells have spread into the lymphatic system.
This is a network of vessels which link different parts of the body - if the cancer has reached the lymph nodes, it is more likely to have spread to other parts of the body.
The breast cancer cells may be tested to see if they are sensitive to the sex hormone oestrogen, and are more likely to grow if the hormone is present.
If this is the case, the woman may be given a drug which blocks the action of the hormone, restricting the cancer growth.
However, as some forms of this drug produce menopausal symptoms, younger women whose breast cancer is more likely to have spread could be offered a combination of surgery and chemotherapy instead.
Following breast surgery, or even breast removal, reconstructive surgery is possible to restore the appearance of the breast.
This could even be carried out at the same time as mastectomy.
Techniques have improved in recent years, and surgeons are able to more closely mimic the appearance of the other breast, giving a normal appearance in clothes.
Some of the techniques employed include the use of implants, fat from other parts of the body, such as the tummy, or even a back muscle which is bent round to form the new breast.