As of 2014, scientists believe Mars is the most likely planet in Earth's solar system, outside of Earth, to support human existence. However, this doesn't say much about the potential for life on other planets, as many major obstacles impede life on Mars without significant technological advancements.
Mars has no oxygen and no air pressure, so special gear would be needed to support breathing. Freezing cold temperatures also would mandate warming functions in spacesuits. The soil is also toxic, which means it is harmful to people, animals and plants. The planet also absorbs high amounts of deadly radiation from space. The best news is access to certain raw materials, such as a potential concrete ingredient called regolith. Another asset is the prevalence of caves to protect inhabitants.