In the past, non-statutory stock options (non-ISOs or ESPPs) and restricted stock grants/RSUs were considered taxable income at FMV at exercise/vesting, potentially creating a tax problem for an employee of a privately held company, as there would be no avenue for selling some of the shares to cover the related tax liability.   Starting in 2018, a qualified employee of a qualified company can postpone recognition of that income for up to 5 years, but no later than an occurrence of a triggering event (e.g. an IPO).  The newly introduced Section 83(i) of the Internal Revenue Code provides guidance.

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