There are two legal bases for inheritance: law and will. Will is a stronger legal basis for inheritance and only if the will does not exist or is not valid it will come to legal inheritance. A will is going to be valid if a testator is able to make the will, if he made the will by his free will, or there were no defects in his will, if it was manifested in one of the forms provided by law and if the contents of the will were possible and allowed. Therefore, a will is going to be exist if there were no reasons for ineffectiveness and voidance. The court shall pay its own motion for ineffectiveness and for voidance at the request of the parties. The reasons for ineffectiveness are the inability of the testator and the inability and inadmissibility of the content. The reasons for voidance are the defects of will on the part of the testator, the lack of legitimate forms and the disposition of the opposite right to the necessary part. The objective of this thesis is to explain the reasons for the invalidity of the will, by stating legal solutions and some examples cases from case-law. In the case-law, there are situations in which the plaintiffs emphasize the reasons for voidance, and the attempt to establish the ineffectiveness of the will. Should the court decide in such situations to repeal the will, it would have exceeded the claim as to determine the ineffectiveness of the will seeks the fact which is different from the one required for its ineffectiveness. Therefore, it is important to pay attention to what the plaintiff's calls for in the lawsuit and what he really requires.