The following table presents the changes in the allowance for doubtful accounts as of the periods presented (in thousands):


	Year Ended December 31,
	2021		2020		2019
Balance, beginning of period	$	1,139	$	514	$	389
Charges to general and administrative expenses	1,163		1,087		282
Write-offs and other adjustments	(730)		(462)		(157)
Balance, end of period	$	1,572	$	1,139	$	514

In addition to the allowance for doubtful accounts, the Company maintains a billing reserve that represents potential billing adjustments that is recorded as a reduction of revenue. The Company’s billing reserve is based on known adjustments and an estimate using a percentage of revenue based on historical trends and experience.

The following table presents the changes in billing reserves as of the periods presented (in thousands):


	Year Ended December 31,
	2021		2020		2019
Balance, beginning of period	$	1,034	$	398	$	440
Charges to revenue	1,780		1,922		516
Write-offs and other adjustments	(1,709)		(1,286)		(558)
Balance, end of period	$	1,105	$	1,034	$	398

Prepaid expenses and other current assets consisted of the following as of the periods presented (in thousands):




	December 31, 2021		December 31, 2020
Deferred offering costs	$	—	$	3,220
Prepaid expenses	5,617		2,848
Other receivables	2,786		873
Other	64		138
Prepaid expenses and other current assets	$	8,467	$	7,079

Property and equipment, net consisted of the following as of the periods presented (in thousands):


	December 31, 2021		December 31, 2020
Computer, equipment and software(1)	$	28,227	$	24,175
Furniture and fixtures	1,118		1,108
Leasehold improvements	3,776		3,708
Property and equipment, gross	33,121		28,991
Less: Accumulated depreciation and amortization(2)	(24,341)		(19,940)
Property and equipment, net	$	8,780	$	9,051


(1)			Includes assets recorded under capital leases of $5.3 million and $6.9 million as of December 31, 2021 and 2020, respectively.


(2)			Includes amount for assets recorded under capital leases of $2.2 million and $2.2 million as of December 31, 2021 and 2020, respectively.

Depreciation and amortization expense was $4.6 million, $3.0 million and $2.3 million for 2021, 2020 and 2019, respectively.

The following table presents the property and equipment, net of depreciation and amortization, by geographic region as of the periods presented (in thousands):


	December 31, 2021		December 31, 2020
United States	$	7,899	$	8,698
EMEA	816		308
Other	65		45
Total property and equipment, net	$	8,780	$	9,051

Accrued liabilities consisted of the following as of the periods presented (in thousands):


	December 31, 2021		December 31, 2020
Accrued bonus	$	3,922	$	2,910
Accrued vacation	3,473		2,724
Accrued commissions	2,633		3,153
Other accrued compensation and benefits	2,474		977
Accrued ESPP	392		—
Sales and other tax liabilities	1,204		1,253

Accrued Professional service fees	647		1,769
Other	3,995		4,653
Accrued liabilities	$	18,740	$	17,439

Other long-term liabilities consisted of the following as of the periods presented (in thousands):


	December 31, 2021		December 31, 2020
Deferred rent liabilities	$	1,988	$	2,543
Deferred revenue	937		1,152
Other	392		327
Other long-term liabilities	$	3,317	$	4,022