It also depends on the motherboard's total RAM support which is divided among all the RAM slots:
For example; If your motherboard's total ram support is 8GB and has 2 slots, or 4GB max per slot, a 8GB stick will not work, and a second 4GB stick would be what it needs. If you don't know this information, post your computer's specifications: If it's a prebuild, post the brand & model and if it's a homebuild, post the motherboard brand and model.