This excerpt taken from the BIIB 10-K filed Feb 21, 2007.
Under the terms of the ESPP, employees can elect to have up to ten percent of their annual compensation (subject to certain dollar limits) withheld to purchase shares of our common stock. The purchase price of the common stock is equal to 85% of the lower of the fair market value of the common stock on the enrollment or purchase date under a look-back provision. In June 2005, our stockholders approved the amendment and restatement of the ESPP, including an increase in the number of shares available for issuance under the ESPP from 4.2 million to 6.2 million shares. At December 31, 2006, a total of 5.4 million shares of our common stock were available for issuance. During 2006, 0.5 million shares were issued under the ESPP. During 2005 and 2004, 0.6 million and 0.4 million shares, respectively, were issued under the ESPP. We utilize the Black-Scholes model to calculate the fair value of these discounted purchases. The fair value of the look-back provision plus the 15% discount amount is recognized as compensation expense over the purchase period. We apply a graded vesting approach because the plan provides for multiple purchase periods and is, in substance, a series of linked awards. In 2006, we recorded compensation charges of approximately $5.2 million.
Cash received under the ESPP in 2006 was approximately $15.2 million.