Background The Norwegian breast cancer screening program started in 2005 inviting all Norwegian women aged between 50 and 69 to attend biennial mammography screening. More than 10 years after its implementation, the high cost-effectiveness of the current Norwegian mammography program has been established by other studies. Objectives This thesis looks at the potential cost-effectiveness of expanding the national breast cancer screening program to include women between the age of 40 and 49 in Norway. Methods and results To reach a conclusion, I first conducted a systematic review on the topic. My systematic review of the topic shows that national mammography programs in western countries that screen women between the ages of 40 and 69 were cost-effective in 7 out of 9 studies (78%). In addition, a six-state Markov model was also developed to evaluate the cost effectiveness of the Norwegian breast cancer program if the program expands to invite women between the ages of 40 and 69 to biennial screening. Cost effectiveness is estimated to be 51313 Euros per QALY gained when comparing the hypothetical program that screens women between the ages of 40 and 69 with the current mammography program that screens women aged 50 to 69. Conclusion With the Norwegian national threshold value being around 700000 NOK(73020 Euros) per QALY gained, it appears that it is in fact cost effective to expand the national breast cancer screening program to screen women between the ages of 40 and 69 biennially for breast cancer in Norway. The results show decent external validity as the calculated expected remaining life years for a 40-years-old Norwegian woman in my model is very similar to the expected remaining life years for a 40-years-old Norwegian woman estimated by Statistics Norway.