For microbiology, agar plates are very frequently used. The dish is partially filled with warm liquid agar along with a particular mix of nutrients, salts and amino acids and, optionally, antibiotics. After the agar solidifies, the dish is ready to receive a microbe-laden sample (although to grow some microbes it is often necessary to apply the sample with the hot agar).
Other Petri dish uses do not involve agar; for instance, cell culture.
Modern Petri dishes often have rings on the lids and bases which allow them to be stacked so that they do not slide off one another. Multiple dishes can also be incorporated into one plastic container to create what is called a "multi-well plate". A microtiter plate is essentially a highly modified multiwell Petri dish plate.
As well as making agar plates, empty Petri dishes may be used to observe plant germination or small animal behaviour, or for other day-to-day laboratory practices such as drying fluids in an oven and carrying and storing samples.
