In this work w e c o n s i d er time d ependent quasiperiodic perturbations of autonomous Hamiltonian systems. We focus on the e ect that t his kind o f p e r t urbations has on lower dimensional invariant t ori. Our results s h ow t hat, under standard conditions of analyticity, nondegeneracy and nonresonance, most of these tori survive, adding the frequencies of the p e r t urbation to t he o n es they already have.The paper also contains estimates on the amount of surviving t ori. The w orst situation happens when the initial tori are normally elliptic. In this case, a torus (identi ed by t he v ector of intrinsic frequencies) can be continued with respect to a p e r t urbative p a r a m eter " 2 0 " 0 ], except for a set of " of measure exponentially small with " 0 . I n c a s e t hat " is xed (and su ciently small), we prove t he existence of invariant t ori for every vector of frequencies close to t he o n e o f t he initial torus, except for a set of frequencies of measure exponentially small with t he d i s t ance to the u nperturbed torus. As a particular case, if the p e r t urbation is autonomous, these results also give t he s a m e k i n d o f e s t imates on the m easure of destroyed tori.Finally, t hese results are applied to s o m e problems of celestial mechanics, in order to h elp in the d escripti o n o f t he p h ase space of some concrete m o d els.jorba@ma1.upc.es y jordi@tere.upc.es 2