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Molecular communications (MC) is a communication paradigm inspired by biology, which employs chemical molecules to convey information. Owing to its tiny transceivers, energy-efficient propagation, and biocompatibility, MC has been regarded as a promising nano/micro-scale communication method that is feasible for information transmission in some scenarios challenging to the conventional electromagnetic communications, such as, inter/intra living cells and other organisms. In this thesis, all the researches are based on diffusive molecular communications (DMC), because of the fact that free diffusion is the most common propagation mechanism in natural occasions. More specifically, the thesis first introduces the motivation and challenges of DMC, as well as the retrospect background and related work of DMC, including channel modeling, modulation schemes, multiple access techniques, interference mitigation, and signal detection. In DMC, inter-symbol interference (ISI) is recognized as one of the important hindrances to the practical deployment of DMC. Therefore, in the thesis, an information modulation scheme called type-spread molecular shift keying (TS-MoSK) is firstly proposed for DMC, which is characterized by introducing extra types of molecules for ISI mitigation (ISIM). In order to mitigate ISI further, two ISIM methods are introduced to the TS-MoSK modulated DMC systems, which are the active ISIM and passive ISIM. Furthermore, the TS-MoSK modulated DMC scheme is extended to support multiple nano-machines (NMs) to simultaneously transmit information to a common access point (AP) or an information fusion center (FC), forming the multi-access type-spreading molecular shift keying (MTS-MoSK) DMC system. In the context of the MTS-MoSK DMC systems, firstly, two detection schemes, namely, the maximum selection assisted majority vote detection (MS-MVD) and equal gain combining detection (EGCD), are proposed to achieve signal detection. To compare the error performance of the MTS-MoSK DMC systems with both detection schemes, the bit-error rate (BER) performance of MTS-MoSK DMC systems is studied based on both the analytical results and simulations. In MTS-MoSK DMC systems, there exist different types of interference, including multiple-access Interference (MAI), ISI and noise, in which MAI and ISI may severely impact the achievable performance. Hence, in order to mitigate MAI and achieve more reliable communications, a range of MAI cancellation schemes are designed based on two fundamental detectors, namely thresholdassisted majority vote detector (TMVD) and EGCD. These MAI cancellation schemes are theTMVD-assisted minimum-distance decoding based interference cancellation (TMVD-MDDIC), TMVD-assisted iterative interference cancellation (TMVD-IIC), the EGCD-assisted N-order iterative interference cancellation (EGCD-NIIC) and EGC-based interference mitigation (EGC-IM). Moreover, following the principle of maximum likelihood detection, a simplified approximate maximum likelihood (SAML) detection scheme is proposed to provide a near-optimal criteria for measuring the achievable error performance. The BER performance of the MTS-MoSK DMC systems employing respectively the considered detection schemes is comprehensively studied and compared. Furthermore, the complexity of these detection schemes is analyzed and compared. Our studies with the MTS-MoSK DMC systems demonstrate that the MTS-MoSK DMC with an effective interference mitigation scheme is efficient for MAI and ISI mitigation and hence, achieving promising performance. However, the MTS-MoSK DMC schemes are relied on multiple types of molecules for supporting multiple-access transmission, which may be hard to implement in some practical DMC environments. To mitigate this problem and furthermore, to address the practical DMC environments where the distances between NMs and AP may be different, a molecular code-division multiple-access (MoCDMA) scheme relying only on two types of molecules is proposed and studied. As different distances result in the 'near-far' effect, the uniform and channel-inverse based molecule-emission schemes are proposed for NMs to emit molecules. For signal detection at AP, different representations of received signals are derived. Correspondingly, three symbol-by-symbol detectors with low-complexity are proposed following the principles of matched-filtering (MF), zero-forcing (ZF) and minimum mean-square error (MMSE), respectively. Furthermore, the complexity and reliability of various detection schemes are analyzed and compared with the motivation to attain a good trade-off. Finally, continuing on the MoCDMA with the motivation to mitigate ISI (called inter-chip interference (ICI) specifically in MoCDMA) and MAI, three frequency-domain equalizers (FDEs) in the principles of MF, ZF, and MMSE are proposed for ICI mitigation. Furthermore, the one/twostage decision-feedback successive interference cancellation (DF-SIC) schemes are proposed to mitigate MAI. Overall, the thesis motivates to propose and investigate the multiple-access schemes for operation in DMC environments and the associated detection schemes of relatively low-complexity. Two multiple-access schemes, namely MTS-MoSK and MoCDMA, and corresponding detection schemes are proposed and studied. The error performance of the multiple-access systems with various detection schemes are comprehensively studied and compared. The studies show that both MTS-MoSK and MoCDMA are effective for supporting multiple-access transmissions in DMC. Depending on a DMC environment, MTS-MoSK or MoCDMA with an appropriate detection scheme is capable of striking a good trade-off between detection reliability and implementational complexity.