Techniques based on spin resonance effects are amongst the most powerful analytical methods in both modern chemistry and medicine. Recent years have seen a trend toward scaled-down detection coils, exploiting the fact that a reduction of the detector size in principle increases the spin sensitivity of the detector [74, 144]. This improved spin sensitivity opens up a new range of applications: In NMR and ESR spectroscopy, the scaled-down detector size allows for the study of mass-limited samples with volumes in the microliter and nanoliter range [16, 111, 157, 162] while in NMR and ESR microscopy applications miniaturized detectors are used to perform high spatial resolution imaging on small objects [6, 10, 122, 136, 139, 156]. Here, the improved spin sensitivity ensures reasonable acquisition times even for spatial resolutions in the micrometer range. In this thesis, fully-integrated and hybrid CMOS detectors for both NMR and ESR applications with varying degrees of integration have been realized and tested. Here, the last generation of fully-integrated receivers, realized in a 0.13 µm CMOS technology, cointegrates a detection coil with a diameter of 345 µm together with a complete quadrature receiver consisting of a low noise amplifier (LNA), a quadrature downconversion mixer and a baseband amplification stage. Thanks to the use of the modern integrated circuit technologies with its scaled-down feature sizes, the receiver is designed to fit entirely underneath the detection coil, allowing for the design of densely packed arrays of receivers. The best measured time-domain spin sensitivity of the fully-integrated designs is 9 × 1013 spins/√Hz at an operating frequency of 300 MHz corresponding to 1H NMR in a 7T magnet. First prototypes of densely-packed arrays in an older 0.35 µm CMOS technology have been designed and tested. In these designs, the size of the detection coils is (520 µm)2. Due to the larger size of the detection coil and the older technology, the measured time-domain spin sensitivities are about an order of magnitude worse compared to the designs realized in the 0.13 µm technology. Apart from the sensitivity tests a first model of the inter-channel crosstalk has been developed and verified which should allow for software-based cross-talk reduction in future versions of arrays. In addition to the circuit realizations, a complete modeling of the signal-to-noise ratio (SNR) performance for fully-integrated radio frequency (RF) frontends for NMR applications has been developed, which includes both the intrinsic coil SNR and the additional contribution from the LNA. The model allows for the selection of an optimum size of the detection coil as well as the choice of the optimum technology and topology of the entire RF frontend for a given application. Apart from the fully-integrated receivers, small arrays of hybrid detection systems consisting of external, high-quality solenoidal microcoils and custom-designed CMOS receivers have been designed and tested. Here, the best measured spin sensitivity is about 5 × 1013 spins/√Hz. In a very specific application for fully-integrated CMOS receivers for NMR, a prototype of an NMR-based positioning sensor for magnetic resonance based surgical guidance has been developed. The microsystem achieves a three-dimensional isotropic spatial resolution of 150 µm in a measuring time of 100 ms. Besides the work on integrated NMR detectors, a fully-integrated frequency-sensitive ESR sensor has been realized. The chip consists of an on-chip LC tank oscillator with a diameter of the on-chip inductor of about 100 µm, a frequency division module and an output buffer. It works in the K-band at an operating frequency of about 27GHz and achieves a spin sensitivity of about 1.9 × 108 spins/√Hz which is the best spin sensitivity achieved using inductive detection methods reported up to now. In addition to the circuit design and testing of the ESR system, a complete model of the SNR performance of frequency-sensitive ESR detectors has been developed and verified against measurements. Here, the noise modeling is of particular interest, because it applies a special case of Bogoliubov's asymptotic method to an LC tank oscillator, resulting in analytical expressions for the phase and frequency noise behavior of such systems which give accurate predictions, agreeing within a few dB with measured results, but furthermore provide the designer with useful insights into design critical tradeoffs.