## 1 Introduction  
Continuous Wave Free Precession (CWFP) is a special regime of the Steady-State Free Precession (SSFP) sequence introduced by [PERSON] in 1958 [1]. The original SSFP sequence uses a train of radiofrequency pulses with the same flip angle and phase, and separated by a time interval (\(\mathrm{T_{p}}\)) shorter than the transverse relaxation time, \(\mathrm{T_{2}}\) (\(\mathrm{T_{p}}\leq\mathrm{T_{2}}\)). In the SSFP regime, the nuclear magnetic resonance (NMR) signal is composed of a free induction decay (FID) after each pulse, and an echo signal preceding the following pulse. When [PERSON] and [PERSON] introduced pulse and Fourier transform NMR in 1966 [2], they studied the condition that optimizes the signal to noise ratio (SNR) in high-resolution NMR. They observed that the highest SNR was obtained when \(\mathrm{T_{p}}\) was shorter than the relaxation times. However, they observed that the Fourier transformed signal shows strong phase and amplitude anomalies when \(\mathrm{T_{p}}\) reaches the SSFP regime (\(\mathrm{T_{p}}\sim\mathrm{T_{2}}\)). In 1971, [PERSON] and [PERSON] [3] showed that the phase and amplitude anomalies were related to the echo component at the end of the FID and they proposed a procedure, called Scrambled Steady State,to suppress these anomalies using a small variation of T\({}_{\text{p}}\) that cancels the echo component. In the same year, [PERSON] [4] proposed the Quadriga Fourier Transform (QFT) to suppress these anomalies. The QFT method was based on a small variation in frequency offset. Although these procedures minimize these anomalies, SSFP sequences were not routinely used in high resolution NMR. We revised these methods and suggested some modifications to increase the applications of SSFP in high-resolution NMR using the Traff apodization function [5] to minimize the truncation problem, or processing the SSFP time domain signal with a parametric method based on Krylov Basis Diagonalization Method (KBDM) [6]. KBDM solves the truncation and phase anomalies but not the amplitude problem. Therefore, SSFP sequences cannot be used to efficiently enhance signal to noise ratio (SNR) in high-resolution NMR. SSFP sequences have been successfully used in magnetic resonance imaging [7; 8] and in low-field, time-domain NMR [9; 10; 11]. The use of a SSFP sequence in low field time domain (TD-NMR) started in year 2000 [12; 13]. The applications of SSFP sequences in TD-NMR can be performed under the drastic condition where T\({}_{\text{p}}\) < 1 ms. Under this condition, a truly continuous wave free precession (CWFP) regime is obtained [13]. Normally, CWFP sequences use T\({}_{\text{p}}\) < 0.5 ms, as T\({}_{\text{p}}\) has to be shorter than T\({}_{2}\)*, which is in the order of 1 ms in most TD-NMR spectrometers. In the CWFP regime, FID and echo signals strongly overlap and are dependent on the precession angle (\(\Phi\)), where the signal is minimal when the interference is destructive, and maximal when the interference is constructive [12; 13]. Under the CWFP regime, the signal does not decay and its amplitude does not depend on the longitudinal relaxation time (T\({}_{1}\)) as in conventional pulsed NMR experiments [12; 13]. All the proprieties of CWFP methods make them an important tool for industrial protocols, as well as in quantitative analysis where CWFP can enhance the SNR compared with conventional CPMG or FID protocols [14; 15; 16; 17; 18; 19; 20; 21; 22]. In this review, we outline SSFP/CWFP theory and their applications in low field TD-NMR to enhance SNR, in quantitative analyses, in flow analyses, in thermal diffusivity measurements, for fast simultaneous measurements of T\({}_{1}\) and T\({}_{2}\), and in the fast T\({}_{1}\) method. CWFP methods have been applied in many conventional TD-NMR spectrometers, such as the Minispec mq20, Bruker (0.47 T) [23], Maran Ultra (0.54 T) [24], SLK-100, SpinLock (0.28 T) [25; 26], Tecmag spectrometer [27], and Oxford instrumentations [28; 29].