The G₀ phase is a period in the cell cycle where cells exist in a quiescent state. Cells enter the G₀ phase at the beginning of what would otherwise be the G₁ phase. These cells appear to have stopped at the G phase of the cell cycle and do not seem to continue onto the other phases. Cells that are permanently in the G₀ phase are called postmitotic cells. An example of such cells are neurons.

Following mitosis, the daughter cells may re-enter the G₁ phase, or a phase called G₀, where growth and replication stops. Cells in G₀ are said to be "quiescent". G₀ cells may eventually re-enter G1 or perhaps die.

The distinction between G₀ and G₁ is not a very good one, and to be honest,in the literature the terminology is pretty inconsistent. Often, this phase is referred to as G₀/G₁. When people use the term G₀, it implies the cell will not be re-entering the cell cycle (will not be dividing). The term is often used to apply to senescent (old) cells, muscle cells, or neurons. G₀ cells have low levels of cell cycle proteins, such as cyclin dependent kinases, and low levels of growth factor signalling proteins such as ras and myc. These proteins are necessary for the cell to traverse G₀ and go on through the cell cycle. G₁ cells are primed to go through the cell cycle, they are just waiting for an appropriate signal to divide. This signal can be a growth factor or a hormone, for example. G₀ arrested cells CAN enter G₁ and go through the cell cycle in some circumstances.