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The NF-κB family of transcription factors promotes the expression of survival, proliferation, inflammation, and differentiation programs in B-cells as well as in other cells of the immune system. Additionally, its aberrant constitutive activation is a hallmark of several B-cell neoplasms. Moreover, recurrent genetic lesions targeting the NF-κB activation pathways have been identified in patient samples from different B-cell neoplasms. For instance, genetic abnormalities in A20, the negative regulator of the canonical NF-κB pathway, and the alternative NF-κB arm TRAF3/NIK have been reported in splenic marginal zone lymphoma (sMZL). In contrast, few genetic lesions affecting NF-κB activation have been detected in chronic lymphocytic leukaemia (CLL), albeit the observed constitutive NF-κB activation in CLL. It has been proposed that signals from the microenvironment might promote NF-κB activation in CLL. Taken together, the recurrent observation of enhanced or constitutive NF-κB activation and the broad spectrum of genetic lesions targeting NF-κB in B-cell neoplasm, strongly suggest that NF-κB activation could act as a general mechanism in B-cell transformation. To date, there is little evidence linking activation of the NF-κB pathways directly to B-cell transformation and there are few available in vivo models addressing the role of NF-κB activation in B-cell lymphomagenesis that bring a better understanding of the disease development, progression and therapeutic strategies. Therefore, the objective of this thesis was to investigate the role of aberrant NF-κB activation in B-cell transformation and lymphomagenesis using available mouse models. First, the potential cooperation between canonical and alternative NF-κB activation in predisposing mice to sMZL was investigated by making use of the published mouse strains mimicking the chromosomal gains in NIK and deletions in A20 observed in human sMZL patients. Second, the direct effect of constitutive canonical NF-κB activation in B-cell transformation was investigated by conditionally expressing an IKK2 constitutive active mutant (IKK2ca) in B-cells, and in the Eμ-TCL1tg mouse model for human CLL. As expected, the hemizygous ablation of A20 cooperated with NIK overexpression in expanding the marginal zone B-cell (MZB) pool in mice. However, homozygous ablation of A20 in combination with NIK overexpression resulted in an unexpected impaired mature B-cell homeostasis that was evident by a systemic depletion of mature B-cells in secondary lymphoid organs and recirculating B-cells in the bone marrow. Moreover, loss of A20 aggravated the previously reported block in adaptive immunity imposed by the overexpression of NIK in B-cells. Aberrant NF-κB activation by ablation of A20 and NIK overexpression resulted in an abnormal pre-activated antigen presenting cell phenotype in B-cells that was accompanied by the altered expression of integrins, important for retaining cells in the MZ. Furthermore, the impairment in B-cell homeostasis was accompanied by the expansion of regulatory CD25+ CD4 T-cells in addition to effector-like CD4 and CD8 T-cells. This T-cell hyperplasia was maintained in aged mice that developed an expansion of myeloid cells with age. The concomitant effector T-cell hyperplasia and later expansion of myeloid cells, suggest a possible involvement of these cells in the observed reduced B-cell numbers. It seems that aberrant canonical and alternative NF-κB activation in B-cells affect the terminal differentiation of mature B-cells forcing cells into a stage that fails to terminally differentiate into follicular B-cells and MZB-cells. Moreover, their aberrant phenotype possibly triggered a T-cell dependent immune response that may be involved in the elimination of these aberrant B-cells. On the other hand, constitutive canonical NF-κB activation by expression of the IKK2ca mutant in B-cells resulted in the expansion of B1a-cells that with age developed into a disease reminiscent of human small lymphocytic lymphoma - a form of human CLL - where CD5+ B-cells infiltrated and accumulated in different lymphoid compartments. Moreover, constitutive NF-κB activation in B-cells shortened mice life span in an IKK2ca dose dependent manner. Furthermore, expression of IKK2ca in B-cells cooperated with the TCL1tg oncogene in accelerating the disease progression in Eμ-TCL1tg CLL mouse model also in a dose dependent manner. Strikingly, similar survival dynamics were observed when the conditional IKK2ca allele was activated during early B-cell development (CD19cre) or in a small percentage of mature B-cells (Cγ1cre or AIDcre). Finally, adoptive transplant experiments demonstrated that constitutive NF-κB activation failed to compensate for microenvironment dependent-signals required for the proper support of CLL-cells in vivo. In conclusion, aberrant NF-κB activation in B-cells had different effects that were dependent on the mechanism of NF-κB activation and transcriptional programs this regulated. While ablation of A20 in combination with NIK overexpression in B-cells impaired mature B-cell homeostasis; IKK2ca-dependent constitutive NF-κB activation promoted the development of B-cell neoplasms in mice and collaborated with the TCL1 oncogene accelerating CLL progression in mice.