A novel scenario is proposed where a scalar field's mass depends on local matter density: it is massive on Earth but free in space. This allows existing gravity tests to be satisfied. The field could evolve on cosmological scales today with couplings of order unity to matter, as expected from string theory. The field's mass depends on the environment, making it massive in high-density regions like Earth but light in low-density regions like space. This leads to the concept of "chameleons," scalar fields whose properties depend on their environment. Chameleon fields can have different EP violations and fifth-force strengths in space compared to Earth. Near-future satellite experiments like SEE, μSCOPE, GG, and STEP could detect these differences. For example, SEE might measure a different effective Newton's constant in space than on Earth, while μSCOPE, GG, and STEP could detect EP violations stronger than current laboratory limits. These results would strongly suggest a chameleon-like model in Nature. The chameleon field's dynamics are governed by an effective potential that depends on local matter density. For a compact object, the field minimizes this potential inside, leading to a thin shell effect where the field transitions to a free state outside. Numerical calculations confirm this thin shell effect, showing that the field is suppressed in the exterior. In the Earth's atmosphere, the chameleon field is suppressed, making it free on solar system scales. Laboratory tests of gravity are satisfied because the field is free inside vacuum chambers. Solar system tests are also satisfied due to the thin shell effect, which suppresses the chameleon force between large objects. The model predicts that future space experiments could detect significant fifth-force contributions in orbit, leading to measurable differences in Newton's constant and EP violations. These results would strongly support the chameleon model.A novel scenario is proposed where a scalar field's mass depends on local matter density: it is massive on Earth but free in space. This allows existing gravity tests to be satisfied. The field could evolve on cosmological scales today with couplings of order unity to matter, as expected from string theory. The field's mass depends on the environment, making it massive in high-density regions like Earth but light in low-density regions like space. This leads to the concept of "chameleons," scalar fields whose properties depend on their environment. Chameleon fields can have different EP violations and fifth-force strengths in space compared to Earth. Near-future satellite experiments like SEE, μSCOPE, GG, and STEP could detect these differences. For example, SEE might measure a different effective Newton's constant in space than on Earth, while μSCOPE, GG, and STEP could detect EP violations stronger than current laboratory limits. These results would strongly suggest a chameleon-like model in Nature. The chameleon field's dynamics are governed by an effective potential that depends on local matter density. For a compact object, the field minimizes this potential inside, leading to a thin shell effect where the field transitions to a free state outside. Numerical calculations confirm this thin shell effect, showing that the field is suppressed in the exterior. In the Earth's atmosphere, the chameleon field is suppressed, making it free on solar system scales. Laboratory tests of gravity are satisfied because the field is free inside vacuum chambers. Solar system tests are also satisfied due to the thin shell effect, which suppresses the chameleon force between large objects. The model predicts that future space experiments could detect significant fifth-force contributions in orbit, leading to measurable differences in Newton's constant and EP violations. These results would strongly support the chameleon model.