Initial standards are higher than continued standards. The ones that bite in practice are the minimum bid price, usually $1.00 measured over 30 consecutive sessions, a minimum market value of publicly held shares, a minimum stockholders' equity or net income test, a minimum number of round-lot holders, and being current on filings.
Falling out of compliance triggers a deficiency-notice and a cure period rather than instant removal. Governance failures, such as losing a majority-independent board or failing to hold an annual-meeting, count too.
Example: a stock closes under $1.00 for 30 straight sessions on 1 April. The exchange sends a notice and grants 180 calendar days. To cure, the stock must close at or above $1.00 for 10 consecutive sessions before the deadline.
Related: deficiency-notice, delisting, uplisting, reverse-split, otc-markets