Description: In estimating mortality rates from the experience of a life insurance company or employee-benefit plan, it is expedient, because of the movement into and out of the covered group on dates other than birthdays or policy anniversaries, to make some assumption about mortality over a fractional part of a year of age. This paper describes and develops the concept of 'exposure' with regard to the more commonly known methods of estimating the rate of mortality, and proposes a fourth methodology. From the Actuarial Research Clearing House 1978, Vol.2.