CONTENTS 0. Introduction.........................................................................................5 1. Basic notions......................................................................................6 1.1. Effective descent morphisms..........................................................7 1.2. Left exact categories......................................................................9 1.3. Factorisations in Lex.....................................................................11 1.4. Descent theorem for exact categories..........................................16 2. The exact completion of the left exact categories.............................17 3. A characterisation of the descent category......................................24 4. A characterisation of the effective descent morphisms in Lex..........30 5. Some further results.........................................................................34 6. The false quotient-strongly conservative factorisation in Lex...........36 7. Conservative morphisms in Lex........................................................44 References...........................................................................................51