Goodwill consists of the excess of the purchase price over the fair value of identifiable net assets of businesses acquired. The following is the summary of the changes to the carrying amount of goodwill for the three months ended March 31, 2021 and the year ended December 31, 2020:

Intangible assets include customer contracts and relationships and covenants not-to-compete acquired in connection with acquisitions, as well as trademarks acquired and software costs. Intangible assets - net as of March 31, 2021 and December 31, 2020 consist of the following:

Amortization expense was approximately $2.4 million and $1.1 million for the three months ended March 31, 2021 and 2020, respectively. The remaining weighted-average amortization period is approximately 3.4 years.

As of March 31, 2021, future amortization scheduled to be expensed is as follows: