The Adventure Kit and Explorer Kits are tested to be leak-proof. The individual silicone lids that come with the system, we are currently calling "leak-resistant." With most higher viscosity testing we have done (think thicker sauces and dips), there is no leaking in high vibration environments. A container full of foodstuff can fall and the lids and contents remain in tact and have performed well, staying on and not lifting off under friction.. With low viscosity, (think watery material), we have been seeing some "spitting" under high vibration or pooling in corners when upside down at an angle. We continue to improve with each new generation. They are currently one of the most robust lids available even still, so we are pleased with the advances, however, we are not done refining.