The two most important aspects of living on the Moon are air and protection from solar radiation. Air is obvious, and requires an entire infrastructure of pressurized habitats, suits, and methods of recycling the air so you don't run out. Future technology may be able to convert other elements into oxygen, but for now, air must be captured after exhalation, scrubbed of excess carbon dioxide, and reused. This requires a huge expenditure of technology and time, all of which need to be in place before the first permanent colonists arrive.
Solar radiation is the other major problem in living on the Moon; because there is no atmosphere, all the radiation from the Sun reaches the surface unobstructed. This can be dangerous for people on the surface, as they can contract radiation sickness from solar flares. Habitats built under the Lunar soil (regolith) are the best method of living without worrying about this radiation.
After those essentials are met, there still remains the problem of food and water, which must either be shipped from Earth, or created on-site; hydroponic agriculture, water recycling, and (potential) ice inside the Moon are key here. After that, the only thing to worry about is bone density; the low gravity of the Moon causes bones to become weaker and can lead to other health problems. A five-year colony would likely become permanent because returning to Earth's gravity well could kill them. Living on the Moon is a major undertaking, and requires planning.