We have a defined contribution 401(k) retirement plan (the 401(k) Plan) covering substantially all employees in the United States that meet certain age requirements. Employees who participate in the 401(k) Plan may contribute up to 90% of their compensation each year, subject to Internal Revenue Service limitations and the terms and conditions of the plan. Under the terms of the 401(k) Plan, we may elect to match a discretionary percentage of contributions. We match 50% of contributions up to 5% of annual compensation. Total matching contributions were $9.9 million, $6.7 million and $4.8 million for the twelve months ended December 31, 2021, 2020 and 2019, respectively.

Under the 2015 Employee Stock Purchase Plan (the 2015 ESPP), amended in December 2019, eligible employees may purchase shares of our common stock at semi-annual intervals through periodic payroll deductions during defined Offering Periods. Payroll deductions may not exceed 10% of the participant’s cash compensation subject to certain limitations, and the purchase price will be 85% of the lower of the fair market value of the common stock at either the beginning of the applicable Offering Period or the Purchase Date. A total of 1.5 million shares of common stock are reserved for issuance under the 2015 ESPP. The 2015 ESPP shall continue until the earlier to occur of (a) termination of the 2015 ESPP by our Board of Directors, (b) issuance of all of the shares of common stock reserved for issuance under the plan, or (c) May 28, 2025.

We issued 59,810, 89,194 and 150,408 shares of common stock under the 2015 ESPP during the twelve months ended December 31, 2021, 2020 and 2019, respectively. As of December 31, 2021, approximately 0.8 million shares remained available for future issuance under the 2015 ESPP.

In May 2015, we adopted the Amended and Restated 2015 Equity Incentive Plan (the 2015 Plan), which replaced our 2005 Equity Incentive Plan and provides for the grant of incentive and nonstatutory stock options, restricted stock, stock bonuses, stock appreciation rights, restricted stock units or RSUs, and performance stock units or PSUs to employees, directors or consultants of the Company. On May 30, 2019, our stockholders approved an increase to the maximum number of shares that may be issued under the 2015 Plan.

We are authorized to issue up to 9.8 million shares of our common stock under the 2015 Plan. As of December 31, 2021, approximately 4.2 million shares remained available for future issuance under the 2015 Plan.

RSU awards typically vest in annual installments over three or four years and vesting is subject to continued service. PSUs are granted to a group of senior officers and the number of shares of our common stock to be received at vesting will range from 0% to 200% of the target award based on the achievement of pre-established performance and market goals. PSUs vest three years from the date of grant, subject to continued employment through that date.

Repurchased shares of our common stock are held as treasury shares until they are reissued or retired. When we reissue treasury stock, if the proceeds from the sale are more than the average price we paid to acquire the shares we record an increase in additional paid-in capital. Conversely, if the proceeds from the sale are less than the average price we paid to acquire the shares, we record a decrease in additional paid-in capital to the extent of increases previously recorded for similar transactions and a decrease in retained earnings for any remaining amount.

We issue new shares of common stock to satisfy RSU and PSU vesting under our employee equity incentive plans. We repurchased 0.8 million shares of our common stock in 2018. No shares of our common stock were repurchased during 2019 or 2020. Treasury stock increased by 43,198 net shares during 2021 as a result of shares received from the exercise of a portion of the 2023 Note Hedge less the issuance of shares to settle the converted 2023 Notes. We have not yet determined the ultimate disposition of our treasury shares and consequently we continue to hold them as treasury shares rather than retiring them.

We have not granted any stock options since 2010. As of December 31, 2021 and December 31, 2020, we had no stock options outstanding. The total intrinsic value of stock options exercised during the twelve months ended December 31, 2020 and 2019 was $7.9 million and $7.4 million, respectively.


		Nonvested RSU and PSU Activity
(In millions except weighted average grant date fair value)	Shares Available for Grant	Shares	Weighted Average Grant Date Fair Value		Aggregate Intrinsic Value
Balance at December 31, 2018	3.2	2.7	$	69.19
Additional shares authorized	2.2	—
Granted	(0.7)	0.7	144.37
Vested	—	(1.4)	69.45
Forfeited	0.2	(0.2)	83.45
Balance at December 31, 2019	4.9	1.8	96.63		$	392.0

Granted	(0.5)	0.5	300.36
Vested	—	(1.0)	92.07
Forfeited	0.1	(0.1)	137.44
Balance at December 31, 2020	4.5	1.2	183.51		430.6

Granted	(0.4)	0.4	351.32
Vested	—	(0.7)	137.90
Forfeited	0.1	(0.1)	279.12
Balance at December 31, 2021	4.2	0.8	$	307.86	$	411.3

The total vest-date fair value of RSUs and PSUs that vested during the twelve months ended December 31, 2021, 2020 and 2019 was $284.5 million, $331.8 million and $207.2 million, respectively. As of December 31, 2021, 0.7 million unvested RSUs and 0.1 million unvested PSUs were outstanding under the 2015 Plan.

Our share-based compensation expense is associated with RSUs, PSUs, and the 2015 ESPP. The following table summarizes the share-based compensation expense included in our consolidated statements of operations for the periods shown.


	Twelve Months Ended December 31,
(In millions)	2021		2020		2019
Cost of sales	$	8.5	$	14.6	$	9.0
Research and development	41.0		37.8		33.5

Selling, general and administrative	63.9		67.0		60.2
Total share-based compensation expense	$	113.4	$	119.4	$	102.7

We value RSUs at the date of grant using the intrinsic value method. We estimate the fair value of PSUs at the date of grant using the intrinsic value method and the probability that the specified performance criteria will be met. We estimate the fair value of ESPP purchase rights on the date of grant using the Black-Scholes option pricing model and the assumptions below for the specified reporting periods.


	Twelve Months Ended December 31,
	2021		2020		2019
Risk free interest rate	0.06% - 0.07%		0.13% - 0.95%		1.72% - 2.55%
Dividend yield	—	%	—	%	—	%
Expected volatility of Dexcom common stock	36% - 45%		51% - 63%		40% - 51%
Expected life (in years)	0.5		0.5		1

At December 31, 2021, unrecognized estimated compensation costs related to RSUs, PSUs, and the 2015 ESPP totaled $157.8 million and are expected to be recognized through 2024.