Reasonable cause defence under section 273B limits penalty imposition, with narrow exceptions preserving penalty for certain failures. Section 273B bars imposition of penalty for specified income tax failures if the person proves there was reasonable cause. The onus lies on the assessee to show reasonable cause for not complying with a notice to produce books under section 142(1); the revenue need not prove deliberate non compliance. Penalties remain leviable despite reasonable cause in limited instances, including refusal to answer or sign statements when legally bound and penalties for income found during searches that is not recorded or disclosed.