Astronaut training describes the complex process of preparing astronauts in regions around the world for their space missions before, during and after the flight, which includes medical tests, physical training, extra-vehicular activity (EVA) training, wilderness survival training, water survival training, robotics training, procedure training, rehabilitation process, as well as training on experiments they will perform during their stay in space.

Virtual and physical training facilities have been integrated to familiarize astronauts with the conditions they will encounter during all phases of flight and prepare astronauts for a microgravity environment. Special considerations must be made during training to ensure a safe and successful mission, which is why the Apollo astronauts received training for geology field work on the Lunar surface and why research is being conducted on best practices for future extended missions, such as the trip to Mars.

The selection and training of astronauts are integrated processes to ensure the crew members are qualified for space missions. The training is categorized into five objectives to train the astronauts on the general and specific aspects: basic training, advanced training, mission-specific training, onboard training, and proficiency maintenance training. The trainees must learn medicine, language, robotics and piloting, space system engineering, the organization of space systems, and the acronyms in aerospace engineering during the basic training. While 60% to 80% of the astronauts will experience space motion sickness, including pallor, cold sweating, vomiting, and anorexia, the astronaut candidates are expected to overcome the sickness. During the advanced training and the mission specific training, astronauts will learn about the operation of specific systems and skills required associated with their assigned positions in a space mission. The mission specific training typically requires 18 months to complete for Space Shuttle and International Space Station crews. It is important to ensure the astronauts' well-being, physical and mental health prior, during, and after the mission period. Proficiency maintenance aims to help the crew members to maintain a minimum level of performance, including topics such as extravehicular activity, robotics, language, diving, and flight training.

The effects of launching and landing have various effects on astronauts, with the most significant effects that occur being space motion sickness, orthostatic intolerance, and cardiovascular events.

Space motion sickness is an event that can occur within minutes of being in changing gravity environments (i.e. from 1g on Earth prior to launch to more than 1g during launch, and then from microgravity in space to hypergravity during re-entry and again to 1g after landing). The symptoms range from drowsiness and headaches, to nausea and vomiting. There are three general categories of space motion sickness:

About three-fourths of astronauts experience space motion sickness, with effects rarely exceeding two days. There is a risk for post-flight motion sickness, however this is only significant following long-duration space missions.

Post-flight, following exposure to microgravity, the vestibular system, located in the inner ear is disrupted because of the microgravity-induced unresponsiveness of the otoliths which are small calcareous concretions that sense body postures and are responsible for ensuring proper balance. In most cases, this leads to some postflight postural illusions.

Cardiovascular events represent important factors during the three phases of a space mission. They can be divided in: