All Phil & Ted strollers have Pneumatic tyres. Flat tyres can be due to puntured or damaged rims. Most commonly it happens when tyres are filled at services station. Although it states on tyre requires pressure of 30 psi in Australia, we do recommand max 22psi .In the warmer months always let some air out to allow air expansion.( The tyre and inner tube can handle the pressure but the rim can't).Don't leave wheels in the hot car or direct sun light.