A famous theorem in the theory of linear spaces states thatevery finite linear space has at least as many lines aspoints. This result of De Bruijn and Erd|s led to theconjecture that every linear space with "few lines" canbeobtained from a projective plane by changing only a smallpart of itsstructure.Many results related to this conjecture have been proved inthe last twenty years. This monograph surveys the subjectand presents several new results, such as the recent proofof the Dowling-Wilsonconjecture.Typical methods used in combinatorics are developed so thatthe text can be understood without too much background. Thusthe book will be of interest to anybody doing combinatoricsand can also help other readers to learn the techniques usedin this particular field.