We study parametric amplification in nonlinear left-handed transmission lines, which serve as model systems for nonlinear negative index metamaterials. We experimentally demonstrate amplification of a weak pump signal in three regimes: with the signal in the left-handed band, with the signal in the stop band, and with the signal at a defect frequency. In particular, we demonstrate the amplification of the incident wave by up to 15dB in the left-handed regime.Left-handed transmission lines are compact systems showing regimes of backward-wave propagation similar to negative-index metamaterials. They have been applied to a number of engineering applications, including the study of leaky wave antennas, compact resonators, and dual-band couplers (see, e.g., Ref.[1] and references therein). In a number of these applications nonlinear elements have been introduced to create tunable structures [2,3] and, in addition, they have been used as a platform for the study of nonlinear wave propagation in the systems supporting the propagation of backward waves [4,5,6].Parametric gain is a nonlinear process whereby a highenergy pump wave exchanges energy with a weaker signal wave through modulation of the material or circuit parameters, thus amplifying it. This effect is commonly used in optical systems, and it has been proposed as a way to mitigate the losses in negative-index metamaterials. In particular, in Ref. [7] parametric gain was exhibited in the left-handed region for a short structure consisting of seven periods with an external DC bias, and parametric generation of backward waves in similar transmission lines was studied both experimentally and theoretically in Ref. [8].Recently, we have demonstrated a bistable regime of left-handed propagation in an asymmetric nonlinear lefthanded transmission line due to the existence of multiple dynamic states [9]. In this Letter, we demonstrate that such structures may exhibit substantial parametric gain in three different regimes of the signal wave: with the signal in the left-handed band, with the signal in the stop band, and with the signal at a defect frequency.In particular, we demonstrate the amplification of the incident wave by up to 15dB in the left-handed regime.Our nonlinear transmission line consists of 20 periods of elementary cells, each cell containing a series varactor diode (SMV1405), shunt inductor (18nH chip type), and a 10 mm section of 50Ω microstrip transmission line, as shown in Fig. 1. The resultant transmission response is presented by a solid curve in Fig. 2. The dispersion relation is of great importance to all parametric processes, * Electronic address: david.a.powell@anu.edu.au and it can be calculated analytically as [1]:where κ is the Bloch wavenumber (in radians per period), L L and C L are the shunt inductance and series capacitance introduced to create the left-handed line;