Let A be a left-symmetric (resp. Novikov) algebra, E be a vector space containing A as a subspace and V be a complement of A in E. The extending structures problem which asks for the classification of all leftsymmetric (resp. Novikov) algebra structures on E such that A is a subalgebra of E is studied. In this paper, the definition of the unified product of left-symmetric (resp. Novikov) algebras is introduced. It is shown that there exists a left-symmetric (resp. Novikov) algebra structure on E such that A is a subalgebra of E if and only if E is isomorphic to a unified product of A and V . Two cohomological type objects H 2 A (V, A) and H 2 (V, A) are constructed to give a theoretical answer to the extending structures problem. Furthermore, given an extension A ⊂ E of left-symmetric (resp. Novikov) algebras, another cohomological type object is constructed to classify all complements of A in E. Several special examples are provided in details. 1 2 YANYONG HONGThis problem is called extending structures problem. The extending structures problems for groups, associative algebras, Hopf algebras, Lie algebras, and Leibniz algebras have been studied in [4,3,6,5,7] respectively. Since left-symmetric algebras and Novikov algebras are important algebraic structures in mathematics and mathematical physics, it is interesting and meaningful to study the extending structures problems for left-symmetric algebras and Novikov algebras. Certainly, this problem is very difficult. When A = {0}, this problem is equivalent to classify all left-symmetric algebras and Novikov algebras on an arbitrary vector space E. Of course, when the dimension of E is large, it is impossible to classify all algebraic structures. In fact, the classifications of left-symmetric algebras in dimension 2 and dimension 3 over C were obtained in [11,10], and the classifications of Novikov algebras in dimension 2 and dimension 3 over R and C were obtained in [12,16]. But, as far as we know, the complete classifications of left-symmetric algebras (resp. Novikov algebras) whose dimension are larger than 3 have not been obtained so far. There are only some papers on the classification of some special left-symmetric algebras and Novikov algebras of dimensions 4 and 5, such as [18,23,24,16]. Therefore, we will assume that A = {0} below. In fact, the extending structures problem generalizes some classic algebraic problems. For example, the extension of the left-symmetric algebra A by an abelian left-symmetric algebra C is characterized by the second cohomology group H 2 (C, A) (see [19]); the problem which asks for describing and classifying the extensions of the left-symmetric algebra A by another left-symmetric algebra B can be answered by bicrossed products of A and B (see [9]) and this problem is called the factorization problem. In this paper, our aim is to give a theoretical interpretation of the extending structures problem for left-symmetric (resp. Novikov) algebras using some cohomological type objects and provide some detailed answers to it in certain ...