The planet Mars has long been regarded as the one world in our solar system, apart from Earth, where life may well exist. The whole problem of life on Mars is examined by a research microbiologist who is also and astronomer and by an observer who has devoted much tome to planetary study. They survey the arguments for and against Martian life; they discuss the surface conditions likely to be encountered; they deal with problems raging from the attractive but improbable ""Martians"" described in speculative works, down to the methods which may be used for searching for Martian organisms as soon as space research techniques have been sufficiently developed. They conclude that the old idea of Mars as a life-bearing world is llikely to be correct, though we can as yet have no real idea of the forms this life may take. Among the topics discussed are the observable physical features of Mars, the atmosphere, theories of the dark areas, possible organisms, life under simulated Martian conditions, the possibility of advanced life, and the problems of human survival on Mars.